Thanks for your reply Dingo.
NBN have replied and are on the case!
We have a splitter in the roof and our second TV is well way from the modem in another room. It suffers the same effects as the main TV.
Yesterday I jumped up on to the roof and changed the direction of the antenna. It made channel 10 clear but made channel 2, 3, & 9 worse. Didn't help 7 at all.
I'll update once the NBN techs have given their assessment.
Regards.
Carl.