Well, thanks so much for all the input, we've put much of it to use already.
We had an .... interesting...chat with DD2 this afternoon, and have more information on the actual situation - apparently there was an element of collaboration from DD1, which we think explains the repetition of the act.
DD1, eventually, blurted out that they "weren't lighting matches, they were practicing putting them out", for which we award ten points for imaginative revisionism.
DD2, or Dante, as we now call her, is as far as we can see chastened by this mornings discovery, and the conversation we had after school struck the fear of god into her with regards to fire, we think.
We belatedly remembered a friend who endured a house fire (no injuries, property damage only, dd2 was not known to be in the vicinity at the time), and she dug out some before and after photos for us, which we saved for the end of the discussion, and I think they helped drive the message home - relating their little pile of matches to what happened to Aunty X's house seems to have made it a little more real.
I called the school, and the head was extremely helpful, and is keen to have a member of the fire service round soon for a general educational talk, and she is going to keep an eye out and an ear to the ground on this topic at playtime - it's a very small school, so I'm confident if anything's going on there, she'll hear about it.
(As far as forest schools go, btw, they look great but there are none in remotely our neck of the woods!)
So, thank you for all the advice, it's nice having here to turn to as my mum would just say "It's a phase", and his mum would say "It's your fault", so a little balance is always helpful!
I think living as we do, where there are regular bonfires on the farm, two open fires and two woodburners in the house, all in frequent, some in daily use, we're not going to adopt any kind of zero tolerance attitude with reference to fire; it's intrinsic to their existence, part of living in the depths of the country, practical application of their knowledge of fire safety will protect them more than avoiding it. We're ramping up the safe and responsible usage message, and keeping emphasis on the fact that fire cannot be abused and is not a plaything, but a tool.
Phew.
So, seriously, has anyone got Ray Mear's number? (letch, letch)
Thanks again all.x