Make of windlass would help. If it is a Lofrans then EC Smith is the distributor.
On the other hand it could be a Leroy, a Lewmar, Simpson Lawrence, etc etc.
Leroy Somer is a huge French manufacturer and windlasses are a minor product for them. Not even sure that they still make them, nor have I ever seen a stockist in the UK. Your best bet is Ancasta who are the main Beneteau dealers in the UK.
You may actually find that the relays are standard industrial items, but I don't know enough about them to advise.
>>> ..... He dismantled the thing and we saw deep pitting on the heavy copper contacts. He cleaned the contacts with a file, and reassembled..... <<
This exactly what happened on our Lofrans relay many years ago. I stripped it down (incredibly simple!) cleaned/filed the copper contacts, and it's been perfect ever since - around 10 years now.
Well worth the very effort to check this out first before reaching for the wallet!!!
If rebuilding the contacts doesn't fix it, just go to your nearest supplier: a solenoid is a solenoid. We sell a particular make whose switches and solenoids are absolutely fine for exeryone else's windlasses and electric winches, too!