Apologies for the delay in replying everyone - I've been away. Thanks for the suggestions!
I found via google an article which mentioned a horn in a tray under the windscreen wipers, which gets full of water and debris and leads to damp getting into the horn. Sure enough, mine was covered in damp leaves. I was short of time so removed the debris and stuffed the horn full of blu tak to muffle it. Turns out this horn is for the security alarm and is not related to the problem (which still happens and is not muffled!).
I've read the manuals - no clues there, but I now think it's not related to the security alarm (and there's no sat nav).
There are no dashboard lights showing, but I will try opening a door while moving to see if it's the same horn.
The seatbelt suggestion is interesting and I'll look into that. And it does seem to kick off if I turn a corner quickly (which made me wonder if it was a dodgy door switch, but now I'm trying to recall if I had anything on the seat).