Hi,
Thanks Nedkelly and Stu038 for the constructive comments.
It turns out the cable is fine - the problem was caused by the faceplates used --
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=13740
With these removed the signal goes back to what it was.
The above faceplates were "unscreened" - would "screened" ones make a difference if I could source any?
I'm sure they would - surely we don't all have coax cable pinned to the skirting boards etc.
Thanks again.
gazbiggs