I see Bob at Checkmate Guns has a new Mk2 with a blue "tigerstripe" laminated grip and a couple of used Mk1 604's in stock as well.
It may be worth giving him a call to see what he is asking for them.
Bob is also good at repairing and servicing the 604 pistols, so you should also be able to negotiate a good waranty from him as well.
Its a bit of a distance for you to travel though for the compulsory "face to face" RFD sale, so a private sale may be better as they can be posted directly to you.
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.