Will call reception first thing tomorrow.
It wasn't from anyone camping around us (by that stage they've all packed up anyway, but it was mostly young families with toddlers in the pitches leading up to ours anyway) (we camping in this trail thing, so a pitch every 10 metres or so, quite secluded). There is however lots of ditches and little streams where groups of children play together. Having calmed down, I can bet you I know which group it was: When we arrived on Saturday morning, there was a group of boys about to start a bonfire in the fire pit on our pitch. (We probably spoilt their fun anyway, as it's a lovely pitch completely sheltered from any other campers, so good for adventure if you're a ten-year old.) As we were supposed to have camped on Friday, it means this would have been empty from 12pm Fri morning. Anyway, when we arrived, dh told them we're about to start pitching there. Nothing, them staying put. Dh: You're welcome to play around here or in stream, but we're going to start a fire here soon ourselves. Them: But we've dragged all this here for our fire, Dh: You can't start fires and unless you want to join us around ours, you will have to go and play with something else. They did leave, although moaning, grumbling, shuffling feet (which I thought fair enough, we did spoil their fun!) Dammit, I bet you it's them. Probably more stupid (as in not thinking) boys than malicious, but still. Dh did try and see if he could see any groups around, but except for a couple of girls playing in the stream, no-one around, so either rocket going far (?) or ran away. Heaven knows, but it did worry me - for example, the three of us had a long nap in the tent from 3pm Saturday, and I dread to think what would have happened if our tent caught fire with us asleep inside... Though, door was completely open, as well as the side flaps, so probably ok. Grr, sorry, am boring you all to tears with this. Trying to get it out of my system, will shut up now. 
PS: Thanks for indulging me and being nice.
It didn't spoil weekend, but it did make me realise that I need to think of other risks too - because we've never camped close to other tents, I've never really thought what I'd do if our tent caught fire from someone else's fire, if that makes any sense, whereas if close to another, there is always the risk that sparks can fly etc.