Ignore this. It wasn't my HP keyboard or mouse at all. It was, in fact, my network card being reported as a 'PS/2 Compatible Mouse'. God only knows why, but after unchecking 'Allow this device to wake computer' in my my network adaptor Power Management setting I have had no more problems, even after turning that setting back on for the HP keyboard and mouse.
There certainly have been events logged where the Wake Source Device is recorded as 'Standard 101/102-Key or Microsoft Natural PS/2 Keyboard for HP Hotkey Support' when neither the mouse nor keyboard were touched, but since disabling the wake function for the network adaptor I have had no instances of this either. All a bit strange, but at least it is resolved for me (for now).
Hope the above may prove helpful for somebody else.