Update

When mixing the silicone with thixotropic I managed to use a ridiculous amount resulting in numerous frowns from other students who needed some thixo and my silicon mould being useless..

But ah well! So I learnt how much thixo not to use by primary bad experience, which is always a good teacher. Also, I went about changing the mould type on Paul's recommendation to a matrix/cavity mould, where you encase the sculpt in a cling-film and then 8mm or so clay jacket, fibrglass over the jacket (in this case two-parts), remove the clay and fill the gap with silicone. So far I have layed-up one side of this mould, but its moving forward and I didn't lose much time overall from the thixo fuck up. And now I know two different moulding techniques..... splendid.