Just a quick update which I'm posting in case anyone else happens to find this thread in the archives with a similar problem.
After following the excellent advise from Milo Minderbinder, which sadly didn't resolve the problem, I was at my wits end googling for hours. I found one thread which simply suggested that removing the power supply from your computer or laptop (or removing the battery from the laptop while its not plugged in) will 'reset' the mobo that maybe having power issues, as Milo Minderbinder suggested earlier.
I unplugged my laptop while it was turned on, then removed the battery and left it out for 5 minutes... hey presto, it's working!
