If the car's broadly OK otherwise, it's likely worth repairing (within reason). That said, I have no knowledge of whether the quote is reasonable or not, but £1000 of welding sounds like quite a lot! I had a premature engine failure on my Saab - an oil pipe failed rather than it wearing out. I paid roughly what I'd paid for the car to have it fixed reasoning the £2.5k was then in effect "buying" me a car with a new engine. 70k miles later this seems like a good decision.
Now that said, at the risk of contradicting myself, I'd probably not spend £1000 on welding up a car which is realistically end of life for other reasons - old engine, rust elsewhere, flaky electrica etc.
As an aside, I thought those Rover 75s were very pretty cars, which a nod to their 60s namesake without being a pastiche. Never driven one, so can only opine on aesthetics