Thanks cp. With there being no clear points to argue on, and being very busy in the middle of a house move, SIL and BIL have decided to pay at the discounted rate. The logbook's not been sent off for a change of address yet so they didn't want to challenge and risk missing a response, but they pass on their thanks for having a look!
Hopefully the up-to-date pictures help for any of the same contravention in the same location also.