Thanks Kitz - the problem is far from solved but it's so frustrating because unless the engineer arrives after its rained and before it dries out then we are getting nowhere! I've taken on board what Newtronstar and Blacksheep say about the warmer weather so maybe it will be a question of waiting until then - hopefully the line will fail altogether
I've read your other posts about the broadcom routers so I purchased an Huawei from an auction site and put it on yesterday. The upstream noise margin is still going mad but it does seem to be holding on to the line better and the bonus is we've gained 2mbps in downstream speed (upstream is poor).
If this does work out better I will invest in a modem/router all in one to save cables etc - is there one that you would recommend to match the Huawei 612 please?