The time allocated is T/O to Landing plus 15 mins taxi.
Agreed, but there seems to be some confusion here; you don't stop the clock between landings - surely this would be very difficult to schedule as each lesson could potentially take hours.
nice new Garmin Transponders
Do you mean the GNS430? That's not a txpdr.
Solo operations at uncontrolled airfields are not permitted under the UK CAA approval.