Interesting, thanks. Because of extensions to by 'bridged tap' extension I could well have been having reflections and loss at multiple frequency bands. I knew about that effect, though I didn't know the term 'bridged tap'. However, the effect on the initial sync was not too disastrous: 58 initial profile vs a later profile after everything sorted of 70.
I had assumed that the major effect which caused drops to 12 or even 3 was having an unfiltered phone on the extension, but I'd be interested in anyone else's more informed opinion. Not too concerned from the point of view of my own line as it is now fine; just generally interested.
Moved (with trepidation turned relief) to BT Infinity 2 for upload speed. Happy BE user for several years.