Just an update to say that the problem has been discovered and solved.

I ended up deciding that I was going to disconnect all the internal house wiring at the incoming master filter to try and isolate the problem. As I gently pulled out the coiled up wires one of the wires fell out of the gel filled connector and both the phones and Internet died completely. I replaced the connector and everything burst back into life, and now the ADSL connection runs for days at a time apart from an occasional reset at 4am in the morning which I assume is something retuning in the cabinet, as after the last couple of resets the maximum upload/download speed has increased slightly each time.

I pulled the connector apart and found there was a small piece of wire behind the crimp so it looks like the wire has broken and just been resting in the connector, and whenever the heat or vibration was enough would break for a very short time, causing the ADSL to reset. The connection was good enough most of the time to not create any noise on the phone line.

I am now having an issue with the wifi on the HG659b modem/router but I will create a new thread for that!