Strangly enough, in a similar situation (though a number of years ago) I sold a car for spares / repair (a whoe £35) without completing the bit at the bottom of the log book. Several months later got a letter re a parking ticket on it. Sent a nice covering letter stating that I'd sold the vehicle (explaining the situation - including the fact the it had been sold for spares and was not in an MoT able condition) with a cheque for the required amount on the understanding I was still technically liable through my own stupidity re the form.
It was never cashed ...