Thinking on this further, Micky.
I think both valuations are correct, if you were buying the gun to have a gunsmith mend it, Phil's £100 would seem fair, but If, like me you'd mend it yourself my £175 --- £200 would be OK.
The leak could be from either the pressure relief valve or the exhaust valve, the pressure relief valve can be accessed by simply removing the action from the stock --- to get to the exhaust valve you need to completely strip the whole gun out.
Imagine Daysdate holding the exhaust valve up and then building the rest of the gun around it.
Either way my valuation is what I and probably a few others on here would be happy to pay for it and mend it ourselves.
All the best Mick