Might it not be a good idea to log GPS data for every lowish-level flight?
With WAAS, altitude data is pretty good nowadays and the recorded track can be compared to the published OS orthometric heights and a good agl plot produced as numerical evidence for every second of flight time.
It's not conclusive proof of height, but it's a lot better than the visual recall of a complainant and it may well be verifiable with extra stuff such as SSR.
Probably best to keep your powder dry and simply tell the CAA that you have contrary evidence which you will produce at Court in the event of an inappropriate prosecution.