These are rare and to Air Rifle enthusiasts of the 1980's important part of air rifle advancement. Not many were made, and custom shops struggled to find enough business though interest in "better" air rifles were there at the time.
The price wall really is: what would it cost to replicate the custom work.
£650 wouldn't be robbing anyone, though to ask much more is asking a lot in the:its rare so must be worth loads of wonga" department. £650 already shows a heady premium for a rifle of its time.
Absolute mint might find a tad more, but mint is more important to Factory rifles. A custom is in reality a factory rifle heavily worked so "second hand" from factory. All the rest is the reputation of the custom maker and some produced real lovelies.
External improvements like lovely walnut are obvious enough. However, its the internal work and advances that changed over time, that were as often the reason these customs were commissioned. Know what done isn't easy to know or find out. Having said that now there are "even better" rifles to be had then what the internal are is less important than just owning the brand custom example. If they shoot well then after so long thats a bonus. Best if not reworked again and have at least some hint the internals had work done all those years ago.
Lastly, part of the premium has to be just how lovely the rifle looks. Much of that is in the eye of the beholder.

If you are happy with the valuation sum then thats good. If you expected more then thats harder. Its what price you can find a buyer at.