Unfortunately the restriction of the yellow lines does apply "behind" them, on the pavement/footpath or verge, to the extent that such is considered to be part of the public highway.
It's not unknown for enforcement officials to be overzealous though, and try ticketing cars on private land. If it appears that the piece of ground on which you parked actually belongs to the house in question rather than the public highway, then I would certainly try to ascertain the precise boundary. I would assume that the owner of the house should know exactly where his land ends.
Even if you shouldn't have parked there, however, all may not be lost. Sometimes the penalty notices are not completed correctly, or the instructions printed on the back about appeals, payments etc. are incorrect, which makes the ticket void.
I would suggest running the details past the guys on this forum, who have a wealth of knowledge about the intricacies of the system:
http://www.pepipoo.comBy the way, sorry if this is a silly query, but I assume you are certain that this is a ticket issued by or on behalf of the local authority and not by one of numerous private enforcement companies operating on private land.