To close this: I managed to grab 1 hour in the attic, and (the OBVIOUS) idea of running most of the distance in RJ45 as opposed to the whole distance in phone wire - doh!
has now given me an 'up' of around 230k and a 'down' of 7-800k on a 'potential' 1mb link, so I reckon I have cracked it!
An extension wifi aerial 'borrowed' from number1 son did not, incidentally, improve the wifi speed on the desktop. Laptop still buzzes on wifi, though.
Thanks again to all helpers.