I would be checking that the fuse is the right rating and then I would check the relay. Things tend to get, as I understand it, when they draw a lot of current. Why the fuse isn’t blowing is interesting. Perhaps there is a direct connection within the inline fuse so look at that. Then look for bad earths, breaks in wiring, general damage to the wires. That sort of thing.
Thanks. The hazards stopped working when left on at the weekend on the hard shoulder, I’ve replaced the fuse but still getting hot. There is a dodgy connection in there somewhere as they wouldn’t start working again without a lot of fiddling with the inline fuse holder (in fact I spent two evenings trying to get them to work again before they sprung back into life).