....as a take off and landing is performed at Bourn Airfield (licensed) just down the road on each lesson
'A' take-off and landing on each lesson? Surely you mean a landing and take-off at Bourn
at each end of the lesson?
One assumes, then, that the student is paying only for flight time between the first take-off from Bourn and the last landing at Bourn since any payment for the remainder of the flying between Gransden and Bourn, whether factored into the charges or not, would make it a public transport flight.
It might not have been the wisest move to bring this situation to the attention of a CAA Inspector, eh Whopity?