If the windows are not in compliance with the reg then a FENSA reg/or
building control cert cannot be obtained.

Correct.


Our man says he usually obtains the building control cert in
preference to FENSA as FENSA only inspect a small proportion of
installs whereas local authority building control inspect all installs
that they certify.

So the bottom line is,comply and therefore get a cert,flout and dont
get a cert.

Our man says that if we dont have a cert,(not in strict compliance)
then this will be picked up by a surveyor acting on behalf of any
future purchaser.

Correct and solicitors are getting ever more tough on this because of
laibilty issues if clients lie.

I guess i'm asking,is the building control cert really necessary or is
it common place to flout?.

Yes and probably.