Okay people, don't give up on me!
I've found what may be a further clue. In Network Connection Details, the 'Lease Expires' time (every 20 mins or so) seems to co-inside with the network disconnections. Now, sometimes I loose connection more frequently than that, but maybe it has something to do with it. What do you think?
I'm pretty certain now it's not my PC or software that is causing the problem... my signal levels appear to be okay, so what else could it be?