The pipe under the hot water tap in my bath is leaking. I've tried tightening the nut but the tap itself moves when I turn the nut. I've tried WD40 on the nut to loosen it from the tap fitting but no good.
Question is, is there any sort of putty/sealant I could use over the nut if I can't actually tighten it?
Not really, you need to undo the nut not tighten it then replace the 3/4" fibre washer. It is not easy i know but with patience and the right tools it will give in. You want a pair of stilsons on the tap, protect it from scratching with a towel wraped round it ,this will then stop you're tap from turning as you work on the nut below with a good quality tap spanner also try a little heat on the nut it often helps.