I hope your logbook was up to date. And do you know why they asked for your licence? Because you need it to carry on working. And I wouldn't worry too much because estimating an aircraft's height is almost impossible, even if you are in the game. But should you get to court (do not accept a caution to get them off your back), they will have to be very clever to get a single ground witness to prove, beyond all reasonable doubt, that you were below 500'. For example, outside the court (don't let the witness know this in advance), ask for the witness to tell you how tall each building is. No errors are allowed. But you will need a lawyer to make your case and make the witness unreliable- which is where your BALPA membership kicks in. You are a member, aren't you!
PM
(an ex-aerial photographer - Aerofilms)