Thanks for the information and suggestions. According to Samknows my home exchange has TalkTalk* and Sky** LLU. Sadly my 2 year Business Broadband contract with Plusnet won't be up till the autumn. I'll be following a well worn path by then. Though Plusnet's service is generally OK, support seems to be getting worse and worse.
A&A seem very determined to honour and preserve their contracts, even where doing so is costing them money (e.g. ADSL on 20CN exchanges).
* While one often reads ill of TalkTalk, my employer supplied ADSL line was TalkTalk Opal. One night I noticed my normally rock solid connection was flakey, and got an ear splitting buzz when I plugged a phone in. Contacted works help desk, who contacted TalkTalk, who contacted OR, who opened a footpath chamber soon after dawn and at least removed a lot of silt from it. TalkTalk line back to normal, and my Plusnet line improved a bit too. Works help desk had been kept fully informed by TalkTalk throughout, updating me in the morning.
** Not that I would touch Sky with a bargepole after my experience with Sky ADSL, where after a 3 month runaround with deteriorating performance (have you checked your connections, changed your filter, done this and that, repeat from scratch on each call) another sufferer with better contacts discovered that Sky knew well that they had a local backhaul problem which would take months to fix, and help desk staff were forbidden to tell the truth. Like Dell (who took my money for an early £1,500 laptop, then gave me months of fobbing off and excuses and only, when I threatened to pass a full record of the correspondence to a computer journalist I then knew, did they admit that they could not build what they had sold me and gave me a refund), they are on my ever increasing "never buy anything from these" list.