If you've lived there 10 years and its obvious (eg. windows stamped
with 2003, brand new bricks etc) that work has been done and you've
put Not Known there may be a problem proving you didn't know you did
it.

The reply is sufficiently ambiguous that they would have to prove not only
that you knew about the work, but that you knew it needed building
regulations approval and that you knew that such approval hadn't been
obtained.

If you just say that you allowed your builder (the professional) to handle
it, or that you didn't really know about what needs building regulation
approval when you did DIY, then it would be very hard to prove anything
against you. (Except the actual failure to obtain building regulations
approval, of course, but this is not likely to be as expensive as lying on
the vendors information form).

Christian.