Thanks to all of you for your posts and suggestions.
To reply initially to MrSaffron, my current ADSL stats are as below (I showed two readings an hour apart). As the speeds vary quite often, they are likely to change quite regularly though.
Operational Mode - G.DMT
Upstream - 800 - 800
Downstream - 2048 - 2016
SNR Margin(Upstream) - 11.0 - 11.0
SNR Margin(Downstream) - 4.3 - 5.8
Line Attenuation(Upstream) - 28.0 - 28.0
Line Attenuation(Downstream) - 52.0 - 52.0
I'll reply to the other issues below in brief:-
* I have changed everything relating to the Internet connection over time, the cables, the router, the telephone system and nothing has seemed to make any difference. Even the phone box has been changed by a telecommunication engineer without resolving the problem.
* There is no cat or other pet here, so we can rule that out as the problem
* I have conducted several quiet line tests and the line seems completely clear, I can't hear any obvious noise or interference.
* I suspect that incoming calls relate to the problem, which is unsurprising. I have seen the Internet disconnect if a call comes in, although it disconnects at other times too. Having the phone unplugged from the microfilter and hence the line seems not to resolve the problem. I tried to plug the modem router directly into the line, thus removing a microfilter completely, but I couldn't find an RJ11 cable that worked. I could use an RJ11 to BT adapter though to try it.
* Nothing was changed on the connection or phone side when the problems started occurring, it had been static for quite some time before that.
Your responses suggest other possibilities I haven't been able to test yet:-
* Having eliminated the router or phone as a possible cause for blowing the microfilters, is it possible that a charge from the line itself is causing the issues?
* The possibility of plugging the modem into the Phone section of the filter was suggested, presumably as a microfilter test. That might be worth pursuing, I'll order an apropriate RJ11 to BT adapter. Does anybody know where I could get a microfilter tester from, or if I could devise some kind of test with a multimeter?
I know this process will take a long time, it already has, it would just be nice to think I'm on the right course and not lost for ideas