Thanks guys!
To remove the tubes from the mandrels use thin walled aluminium tubing for the mandrel, expands more than steel & plenty of release agent. Once the tube is "lubed" up as it were its actually very hard to get the first layer super tight. I have only had one tube that seemed to be stuck on the mandrel so far. Whacked that one in the freezer for about an hour, then with a little pressure I managed to twist it off.