Thanks for the information. Yes it was the aerial. The recent high winds (and a rusty pole which was crumbling and made the fastening a little loose) had deflected the aerial quite some distance
Both my Winter HIll and Storeton aerials are rather old (Winter Hill being around 30-odd years old and the Storeton one about 12 years old). The pole was, I think,installed when ITV came out around 1956 so it lasted quite well :).
All signals now at 100% on both aerials after the local aerial man (who I've known for over 20 years) replaced the rusty pole, checked everything over and refastened/repositioned the aerials.
He advised "if it ain't broke, don't fix it" and that both my aerials are quite suitable for use and working well at 100% signal and strength.
Thanks anyway. I'll bear you in mind as he is getting near retirement age now.