Just to let everyone know how this ended up.....
SNR continued to plummet from anytime from 1pm onwards every day.....no pattern to it....sometimes 1pm, sometimes 3pm, quite a few 4pm's but 90% of the time the 'interference' kicked in on the hour and just got worse as the evening went on. Eventually I was dropping through the first 15db of SNR, resyncing, then dropping through another 15db of SNR and resyncing at a very low rate. Even then, it would drop through another 7-10 of the next 15db margin straight away! Finally after all that, my sync rate was lower than fault threshold so I reckon I had a good case.
Contacted my ISP (Nildram - who have been excellent), who immediately spotted the line was quite 'erratic'. The first guy I spoke to knew all about SNR and instantly knew what I was talking about. Reported to BT as a fault. BT tested line and report nothing wrong. Nildram persist and goes to another team in BT who decide need engineer visit. BT engineer visits, see faceplates etc.....'looks like you have tried everything already', does a few noise tests and leaves to check exchange thinking that a 'lift and shift' may be the best option...broadband will be off for approx 30ins while they do it.
Don't hear anything for a few days......adsl continues....no sign of a lift and shift.... then adsl signal disappears from line and doesn't return. Leave it for 24 hours before contacting them. The 'lift and shift' didn't work?!?!? so a cease and reprovide is needed. A few days later adsl returns. Now a lot more stable than before. SNR falls by 5 or 6db with darkness which is more normal. Line has been syncing at 5728kbps for last 48 hours without any issues - which isn't bad for 47db attenuation. So hopefully, problem solved. Its just a shame BT can't tell you exactly what the issue was! Can't fault the ISP though.
Brian