Thanks for the input from one and all - apologies for the delay in my response. Initially I was watching the thread - then I had to go away for a while.
The departing aircraft (on THIS occassion) was a Beech 1900 (OK, OK, a turboprop, not a "jet"). The arriving aircraft was another Cessna trainer that had been cleared to land some time before we came on tower frequency.
Student was handling the aircraft as part of a check ride...and he took the controller instructions and crossed the hold short line in the bat of an eye.
I did take control when airborne and turn upwind - whilst student was explaining to me that we would 'rotate before the Beech did'. We had some post flight discussion about relative rates of climb.
The same scenario has happened again, since, and this time I was ready for it. Refused the line up clearance.
No - it was not in the United kingdom....as I'm sure you all knew.