Hi. Thanks for the reply.
Yep, the effect is repeatable and it happens with both phones in the house, a DECT phone and a conventional wired handset plugged into the master socket.
I've tried the dial tone suggestion by ringing in and letting it ring before answering, strangely enough the downstream SNR rises about 1 db while ringing in but then drops about 2 db again once the phone is answered.
(I'd hoped to post a trace from DSLStats but have given up for now!) Ah! ...
On the first drop I picked the phone up straight away, on the second I let the phone ring first.
Cheers - Andy