Just a quick update for anybody who's been following this.
The problem now appears to have been sorted - faulty equipment at the exchange was the culprit apparently.
I've been critical of the customer service I've received in the past, but the last month has seen a real turn round in the support and help I've received.
I'd like in particular to offer my thanks to [mremoved], the Plusnet staff member who finally sorted this out. He was determined and dogged in pursuing the fault with BT, kept me regularly updated and was really helpful all the way along. So, well done Plusnet and thanks again Shawn!
jim:csa