I asked Simpson Exhausts about getting something made after seeing the one fitted to Harry Metcalfe's Sport, they said they would need the car there to make an exhaust. Might be worth contacting but probably similar price to the alternatives already mentioned.
I have a pattern Evolution engineering 4 part ported for 1600 that I can reproduce. Obviously not homologated but works well and ties in with existing silencer system.
I could get interested in there were say 5 people wanting them c£1400 in stainless 308