I've bought Walther parts from Judith Billharz at www.SchiessSport-Billharz.de. She's based in Germany but I think she's still despatching parts to the UK. The 15J 400 regulator is exactly the same as the walther Dominator 300 regulator but I think you will also need some type of spacer. I don't know whether the 15J 400 uses a different hammer or hammer spring to the 7J set up, the best thing is to probably ask her what you need for what you're trying to achieve. There could be some complications with the recoil compensator too.