You have a choice.
compression bent 409L stainless or Mandrel bent 308L stainless

Do you know what the difference is between £200 and £300 

(see below for the answer)

MANDREL BENDING 
Consider a straight plastic straw that you might use to drink beer out of a bottle. If the straw is completely straight, it's very easy to draw the liquid out of the bottle, as there are no restrictions to the flow of beer. However, lets say you decide to bend the straw 90 degrees into an "L" shape. Now, drinking from the straw will be very difficult, as the opening at the straw's elbow will have changed from a circle to a very narrow slit. This is exactly how the pipes in your car's intake and exhaust system work. Air is drawn through pipes, which, when "compression bent," become very restrictive to the flow of air.Mandrel bending is the solution to this problem. Mandrel bending works much like the flexible straws that you can buy in a grocery store. When the straw is bent, a flexible section of the straw expands to maintain the round opening no matter how much of an angle the straw is bent at. When an exhaust or downpipe is manufactured with mandrel bends, the steel is allowed to stretch on the outside of the bend and compress on the inside of the bend, maintaining the nominal pipe diameter.

PERFORMANCE EXHAUST
Zorstec is one of the few exhaust manufacturers in The UK with a mandrel bender. Ever wonder why every exhaust manufacturer doesn’t offer mandrel bending? Simple, the equipment & tooling required to mandrel bend tubing is very expensive. We at Zorstec have a commitment to offer the absolute best in exhaust systems at the most affordable cost possible. Our custom made tooling allows us to produce a product that will give you the most power possible.

PIPES & POWER
Pipes are critical to high performance. Zorstec offers mandrel bends for better flow versus stock headers. stainless steel gives them a longer lasting sharper appearance.

MANDREL BENDING
The improved flow that comes from a mandrel bent pipe makes Zorst exhaust systems superior. The mandrel bending process maintains a constant inside diameter, even through pipe bends. Flow area remains consistent, increasing flow more than 35% compared to stock, serrated bent pipe
Other than something like a rubber hose, it can't be done with bare hands. It will kink, crimp, collapse, or break. Small, malleable metal tubing, such as brake or fuel lines, or electrical conduit, can be hand-bent with special tools that cradle the tubing in a trough the same size as half the outer diameter (o.d.) of the tube.

compression bending
In the early '60s, a man named Huth invented a machine to bend exhaust tubing in the same manner. Using a pair of half-round concave steel dies, and hydraulic power, it could quickly and easily bend pipe up to 90 degrees, possibly more. Before this, anyone building headers or exhaust systems had to cut pieces of pre-bent tubing and weld them together to get the shapes and angles needed to snake the exhaust system through the chassis. This, as you can imagine, could be quite time-consuming. The Huth-type hydraulic tube benders have become the staple of tube bending shops everywhere. They're fantastic for what they do. They revolutionized the exhaust industry.

compression bending or mandrel bending
The only problem is that with this larger-diameter, thin-wall steel tubing, even the half-round dies cannot bend it without collapsing or crimping it to some extent. It's fine for regular automotive exhaust systems. But for high performance, any reduction of pipe size creates a restriction which, in turn, reduces power and economy. The whole approach to Zorstec is increasing airflow into, and out of, the engine. A major means to this end is increasing the size (diameter) of the tubing in the intake and exhaust systems—primarily the exhaust. Any exhaust system, especially on long vehicles like big pickups or motorhomes, will require several bends. The larger the tubing, the more severe the crimping will be with regular tube benders. Shrinking the diameter of the tubing at each bend negates the benefit of going to larger-diameter tubing.So the solution. It's called mandrel bending. It's a bit more costly and time-consuming, but it's very effective. A mandrel tube bender is very similar to the Huth-type in that it uses a pair of half-round steel dies to hydraulically bend the outside of the tubing. The difference is that the mandrel bender inserts an articulated steel "stiffener" (the mandrel) inside the tube to keep it from collapsing. It's sort of like a series of three or four metal donuts (held together by a ball-and-socket and spring arrangement) just a hair smaller in diameter than the inner diameter (i.d.) of the tube. It's really a pretty simple system that has been around for quite a while.

What is the difference between mandrel bent and pressure bent? What does CNC mean? 
Most factory exhaust systems built through the 70’s were pressure bent. Pressure bending is when a tube is placed into a die and a hydraulic ram, which has the other half of the die, or the shoe, is then pressed together to push the tube around the radius. Nothing is inside the tube to prevent the tube from collapsing or buckling, and the machine is completely manual-the operator indexes the pipe by hand according to an angle gauge in the end of the pipe. Not a very precise method of bending, but quick and inexpensive.

Mandrel bending is when a set of balls, called a mandrel, is inserted into the pipe during bending. These balls are positioned and pulled through the pipe so that the bends are not deformed and maintain a close to perfect shape throughout the length of the bend. The pro’s are that the tube can be bent on a much tighter radius and still flow well due to the pipe being round and not collapsed. The machinery is expensive for a good quality CNC mandrel bender with the appropriate tooling. 
CNC bending is when a computer controlled set of motors indexes the head of the machine and controls the position of the tube and the rotation of one bend in relation to another. Essentially, the machine does all the work. The tube is cut to the appropriate length, inserted in the collet, and the button is pressed to start the cycle. The operator does not touch the part until all the bends are complete. CNC mandrel bending is the best method for producing exhaust systems that flow and fit well. With a CNC mandrel bender, you can also bend thin wall tubing so that it weighs less. Being stainless, it will not corrode so thin wall tubing is not a concern for durability.

Why is Stainless better than other materials for exhaust systems? 
Stainless is an excellent choice for exhaust systems due to its corrosion resistance. Starting in the early 90’s all OEM manufacturers went to 409 stainless for the increased durability and corrosion resistance-the EPA mandated that exhaust/emissions systems be warranted for 5 years minimum.Zorstes 304L grade stainless will not rust or corrode over time. Unlike aluminized steel, it will not rust from the inside out due to the corrosive fluids produced during combustion. All of Zorstecs Stainless 304 systems have a lifetime guarantee against defects in materials and workmanship.

Different grades of stainless?
What is the difference between grades of stainless (304, 321, 409…..?) 
Without getting into the specifics of metallurgical properties, 409 is what is typically used on OEM applications. It has fair corrosion resistance, is magnetic due to the high metallic content, but is a less expensive material than 304. 409 is not an ideal material for a performance/restoration exhaust system because it will turn color (brownish/rust) from being exposed to the atmosphere without ever being run. For OEM’s who never care what the exhaust system looks like, it is a decent material that will last long enough to get past the warranty period. 304 stainless has much better corrosion resistance and a lower metallic content than 409. 304 will not turn color from sitting exposed to the atmosphere, and will only discolor slightly from getting extremely hot. The best thing about 304 is that you can maintain it with a scotch bright pad if you ever spill oil on it or have it discolour. Zorstec only uses 304 stainless, which is a premium mil spec grade of stainless that is more expensive than standard stainless inwhich is processed much more closely and is more consistent.

so £200 every 3 years or £300 once