Today it got so bad that I only got a couple of photos taken while up in the Lakes, back home checking on the net on causes of the symptoms it seems this is a common problem on alot of Canon models related to the battery contact on the case opening not getting enough contact with the batteries.
My first attempt at fixing my camera was to bend the connector that goes to -'ve battery, this ended up making it worst as it would only turn on and stay on when the camera was upside down. But it did prove that battery contact was the problem as the red low battery indicator stopped coming back in my tests when it was upside down and I could tap the camera and it would turn off.
To fix my problem I ended up having to pull the -'ve connector right out of it's slot, as shown below so it's in constant contact with the battery.
I showed my fix to my dad who has the same camera, below is how the battery sits in his camera. From the picture you can see that the battery is 1-2mm higher than it is in my camera, so it looks like although I've apply a working fix to the contact on the case lid, the actual problem has occurred with the +'ve connector at the other end inside the case - it's lost it's spring feedback.
I'm leaving the fix as it is, will be over the top to take the camera apart to fix the source of the problem.
Update Sept 2011 - Thanks everyone for their comments, I really appreciate how useful you found this post, I've recorded a video (below) of how to do the fix. You'll see the first few attempts didn't resolve the problem and rather that cut them out I thought I'd keep them in so it's clearer what finally needs to be done.