It may have just been coincidence I don't know, but I followed the steps of a previous poster in this thread, supposedly from a HP technician, to remove the mains and battery, and then hold down the power button for 30 seconds. It actually worked for me, so thanks for that post!
Just wanted to repost this in case anyone else missed it earlier in the topic. Hope it helps any others in the same boat, because I know its frustrated me for days!