The landline telephone works (but instead of continuous has an intermittent tone with approx one second bursts) but the moment I plug the internet router back into the ADSL splitter (plugged into main BT socket) the phone goes dead, even with the router unplugged. The internet has not worked for several days and I am trying to trace the problem.
>>> instead of continuous has an intermittent tone with approx one second bursts
Are you sure that isn't just the 'message waiting' tone, which is a two-tone sound (a bit like the old police sirens but slower). If it is, call 1571 to retrieve the message. Then see if things have got better when the dial tone returns to normal.
Also be aware that ADSL splitters can pack up after time, resulting in connection problems. (Written from experience; I've gone through several over the years!)
Further to Buenchico's comment. You can easily check I'd the problem is with the splitter, by simply plugging your router directly into the master socket. If you can then connect to the Internet, the problem is the splitter.