Good afternoon, Paul...
Ah, memories of my Watkins Westminster amp, over half a century ago..! Happy daze..!
As these amps are now very collectable, I think I'd recommend confiding the amp to a specialist who knows what he/she is doing, and respects the vintage nature of the amp. I can't see your location, but I have two addresses (there may be others...) that are experienced in this field. One in Nottingham (Champ Electronics, hoping he's still working..!), the other in Wiltshire;(Vintage Amp Services...). They can get your amp back to its best performance, maintaining its value as much as possible. Either would, I'm sure, be able to estimate repair costs.
The alternative would be to sell the amp 'as is', hoping that the buyer understands that it's a vintage amp, with all the foibles of its age, and will, in any case, need to be attended to.
If you intend to keep it, of course, it's a 'no-brainer' : get it fixed by the experts for another fifty years of loyal service.
Hope this helps.
Douglas