Thanks BdB, I've learnt a couple of things that is best not to do, one is if you have a sharp edge when you start, you'll have a sharp edge after you have done the mould, and the sharp edges really did a number on the mould, especially when I was trying to get the new cast bit out of the mould. Secondly, not to try and do everything in one mould, some things will have to be done in a two-part mould, which is something I'll have to practice with.
Robert