The only thing I've had work is to take a knife, putty knife, or pie server and start working around the edges until it will come off. If it hasn't been set for a long time, it will probably come off easier.

Good luck!

My mind must have gone blank for a moment as your very simple answer should have occured to me. Thanks it worked like a charm :)

That's what I do also. Make sure you get ALL the old adhesive off because nothing will stick to silicone after it cures. I scrape, then sand to make sure it's all gone.