I think Ive just fixed it, although not entirly confident of how long it will stay fixed.
I think its the nut where the pipe goes into the cistern (Ive put arrow to it)
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I cant get a spanner on it, so I tried tightening using my fingers. Of course, I turned it the wrong way, so what little water there was in the cistern started to leak out. So I quickly turned the nut in the other direction and it stopped. I could barely feel it turn at all - I can only get two fingers on it - but its stemmed the leak. Ive flushed the toilet a couple of times and so far no drips.
Ive put some toilet paper underneath and will keep an eye on it to see if it starts dripping again.
Could be as simple as the nut working its way lose by a tiny tiny amount? Is this normal/usual? Because Ive had to tighten in using my fingers, with my wrist at a really awkward angle, Im not confident Ive tightened it enough for it to stay fixed.