Nigel
As I said they are available new, we use them on all our modern Walthers (old school!) they are "Kornhalter M18 article number 2.4827" and 49 Euro's. I don't have one spare (just replaced the Centra one on my LG400 with one), Walther will not supply direct but I can give you a contact German company who will if you want one.
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?