Could this be the little...ahem...'incident' at Elstree late last year??
If I remember correctly an instructor let his student fire a Cessna up while he went and finished his coffee. The student had the throttle set too high, didn't have the park brake on nor his feet on the toe-brakes and just panicked when he shot forward into the rear end of another aircraft.
Its a stab in the dark but looks like the same incident to me...
Never ever forget to brake,
SB22