I can try another game at the weekend and let you know.
There are no other issues as far as I can tell. Discord still works fine at that time, as do websites, and speed tests. Everything works fine the rest of the time.
Its installed on an SSD with plenty of free space, so it's not an FPS drop, more like a packet loss issue, but only during busy times.
It's on a wired connection.
Thanks
EDIT: I possibly should add that at the best of times my ping to the server is about 115, and at busy times it goes to about 140. The bad evenings where the game becomes unplayable it is still at about 120-140. If I didn't have a powerful computer, and if it didn't work fine at other times I'd have thought it was a PC issue, rather than connection.