Araldite 5min epoxy.

[JBK]

so using the heated blade takes it off nicely. the area where blue-tack is wasn't glued

[Faceting Frank]

If you heat the dop with a BBQ lighter, the stone can be pulled off by hand. Steeping both stone and dop in acetone will soften the glue and it will peel off without risking damaging the stone.
When I was talking about Blu Tack before I didn’t mean putting any on the dop to be glued to the stone. I meant holding the stone with it on the other dop in the transfer jig, so the preform is sandwiched between the two dops. It can then be accurately aligned, the dop slid back slightly to get some glue between the dop and stone, slid back in place and glued up. 

[JBK]

Right, cheers Frank. I've misread it - thinking it would protect the facets against the cone dop direct contact when taking the stone off.

[Faceting Frank]

I found out from the net that the workable temperature for Araldite 5 min epoxy is between 5 and 30deg, so now I understand why sometimes I was having problems. Dopped a stone yesterday after putting the tubes in the fridge for half an hour, that worked a treat. So, it does not matter what the tube storage temperature is, as long as the mixing temperature is ok.