Barry, if you're going to run a cable anyway, might you not be able to put in a proper ethernet cable?
I need the new telephone connection to get ADSL to the router's new position.
However I managed to get one of those 'evil' ethernet couplers and, by joining two cables, could then put the WAP in it's new position and connect to the router.
From my Wife's machine it was getting a signal of around 42% and a strength of -59 dbm and she said that internet sites were displaying as fast as she would normally expect so it seems that it should be OK.
So 'all' I need to do now is to grovel around the attic to lay the new telephone cable, move my stuff downstairs and connect my machine via the Solwise HomePlug devices and all will be hunky dory.
I was able, in a test using my laptop, to get around 30 Mbs over the HomePlug link.
Thanks for everyone's help. I have probably learned something from this experience but the problem is that I won't be able to remember it when I need it again. :-((