Orders started being rejected again, despite obtaining a 3rd MAC, but one of the Plusnet support guys took ownership of the problem. Eventually BTW admitted there was a glitch in their MAC handling procedures. 12 days later they reported that it had been fixed, Plusnet re-ordered FTTC for me and the order was accepted.
The installation took place this morning. The engineer was here for 45 minutes and didn't want a coffee. All is working well - throughput with tbb speedtest is 35.5 Mbps.
So for those of you who have had orders rejected for inexplicable reasons, try again now and you will probably be lucky.
Plusnet Value Fibre, Netgear WNR 1000, St Ives Cambs