Hi, dencorso,
This isn't necessary the information that Yellow Horror give was definitely the cause of what was happening - that and that I had a Windows setting made that allowed internet activity to wake the computer once it had gone to sleep (as detailed in posts above). Basically, I had one router, a Virgin Media Super Hub 2, connected to the computer in modem only mode. Seems that when this is done then, as Yellow Horror correctly identified, the router only acts as a bridge and hence has no firewalling functionality (in that mode) and hence my computer was exposed to all internet activity aimed at it and that was what was waking the computer from sleep. If I then connected another router between the Super Hub and the computer the waking from sleep problem disappeared -- the second router was now providing firewalling functionality. Hence, the problem was using only the Super Hub in modem only mode which was exposing my computer to all internet activity aimed at my machine and that was what was waking it up.
Thanks for the thought though.