I want to make a 2-part mould roughly covering 3.5sqmtrs but the overall cost is quite large.

Would it be possible to use a cheaper alternative to the Uni-mould Tooling resin (which appears to be the most expensive part) such as a cheap Polyester resin to bond all the layers of chopped strand mat? Or is there another cheaper alternative to the uni-mould system that would give adequate results?

I don't intend on pulling hundreds of releases, at most I would say 5 in total?

Excellent stuff, that has reduced the overall cost massively! I think I would prefer to use a proper tooling gelcoat but skip the coupling coat and use polyester to bind the CSM.

What are the advantages of using the full system i.e. all three stages of the uni-mould? Seems like a much more expensive way of doing it!?

A rapid tooling system like Unimould will produce a much better mould than regular GP laminating resins. The main advantage is time, it is very quick and easy to produce a tool because you can layup the bulk reinforcement in one go. You can do a complete mould lamination in a few hours and have a ready-to-use mould 24 hours later. This is more important for professionals than hobbiests, but still a great advantage to have.