Yep, the trials of casting. As you say mixing thoroughly and carefully (bubbles) is the key to a good cast. You might also consider using spray wax instead of Jelly (they sell it in the stores at the Uni). If you get more problems bring it in and we'll cast it here - Andy or I can help.
Thanks Alan, I've already re-casted the head and it worked perfectly the second time around, I'm just trying to catch up with documenting everything into PDFs. I've also almost finished casting the head into silicone but I ran out of catalyst last week so I've been waiting for my next delivery of supplies whist writing. Once Dissertation is handed in I'll be able to post up a bunch of updates :)