Thanks again for the very useful advice.
I am living abroad at present so phoning PO is not too practical, but I am in contact with PO via email and gradually building up documentary info from them to use when I return to the UK in a few weeks time.
Assuming that a full refund is possible, which off-net broadband providers should I consider?
On a related subject I have looked at http://www.coolwebhome.co.uk/cds-map/
for Devon and it shows a group of EX5 post codes near to Newton St.Cyres (Exch/Cab) at which superfast broadband can be expected. I live about 1 mile from that exchange and according to samknows.com that exchange can only provide ADSL Broadband.
I have done speed tests during the daytime and it is always below 1 Mbps. Download speed early morning before sunrise seems to be faster, but I've never checked it.
Can you explain what the situation is at this location?