I have a small background doing some fibreglass repairs and mould making so have a slight clue about what I'm doing. But obviously not too good as I'm having a nightmare here. 

To be honest though, I think for people without a lot of experience, semi-permanent release agents can be a bit tricky for making moulds. They are excellent and give the best results, but they can be risky in themselves because you need to be meticulous with your application and then they do make it more likely for problems like gelcoat separation and pre-release. Not to mention you need to be 100% sure that the surface of the substrate you are moulding is compatible with both the release agent and the tooling gelcoat. The most reliable release system in my opinion is PVA. It will never give you a perfect surface so you will always need to do some refinishing work, but if you spray it on then you will be close and you can be absolutely sure of a release.