So, just as a bit of an update. For the last couple of days, our connection has gradually got worse and became noticeable. We checked our equipment and found no issues, we changed to a different modem, cable, tried the master socket and no difference.
According to Zen, the line was fine and there was no issue, it MUST be our equipment.
Then yesterday, a BT van mysteriously appeared at our cab. Our connection got more and more flaky, then dropped at about lunch time. When we came back from lunch, we rebooted the modem and hey presto:
I think there's still something going on but for now our connection is stable. I noticed a huge spool of cable outside this morning so I'm hoping they're laying fibre (Our exchange is listed as "Coming Soon" and has been for a while now).