Can't multiquote on my phone, but cheers for both the suggestions.

Mr S1 - have tried that already to no avail. Guess I could try and file it a bit, as it's currently a bit big for the next socket down, even with a hammer. Also quite tricky to get access.

Will - might try cutting to flats on it. Another idea was to weld a nut on to it? That seems like quite an easy option, and I know plenty of people who weld.