Thanks for all the replies folks. Altis those links are just the job. It confirms that 687.25 MHz is the correct frequency for channel 48. When C5 first went on air I had problems with the picture long before I had Ntl Digital. BBC1 to C4 where perfect yet C5 was poor. I eventually traced the fault to a poor coaxial connection. As Martin says that could be the cause of the fault once again as all the cables have been disturbed for the new TV.
Jim.