Had a walk round the village.
On my return, entering the street off which we live there was an OR van parked with a guy banging down covers on an inspection chamber. I asked him if he had been working there all day and if what he was doing would account for the ADSL disconnects. Answers: yes & yes.
Fingers x'd that once they have stabilised the cable splitter degradation there the connection will return to the normal stable situation.
The unchanged SNR had already lead me to suspect a line rather than an exchange fault so it was good to be proven right.