Well I had offered to take them flying - they then offered to pay for half the cost! Seemed like a good idea - and since they're air minded young folks I said I'd talk them through what I was doing.
Certainly not my intention to give instruction, but rather the experience, in the same way that I would if say a relative or friend wanted to go for a flight & would contribute to the cost.
Since I'm not recieving renumeration for what I'm doing - just costsharing - I was under the impression it was all ok.
But I will take your advice and re-check before I do anything.