Sounds like a fault somewhere on the line, have they got any unfiltered telephone extensions/tried removing the bell wire from the master socket/ tried different filters, if all that fails I would try rining their ISP and ask them to do a line check.

Thanks, will check those things out.

(I think there's an extension, but no idea if filtered or not.)

My mum has just mentioned that BT are doing things to improve broadband in this area, so might be related to that.

Sounds likely to me, they usually mess something up when they tinker.

When it has a spat, if they try clearing the NAT table for it, does it start working again? I had one of those (although I think it was the GT) that would fill it's NAT table and then fall over. Touch wood, my other identical model is still running fine.