Alan here from sunny Barnsley lol
I'm making a mould for a air intake to the air box on my Ford Focus RS its an odd shape and hopefully you could help me decide how many splits on the mould I would need ?
I need to sand down the joints then cover with filler before flatting then I plan to spray it to get a really good finish 

No problem :-). The main trick is to make sure that you don't get a mechanical lock. There are a couple of ways to achieve this. The traditional one is to make sure that any parallel sides of the mould sections aren't exactly parallel, they actually move away fro each other very slightly. An alternative method that I have used is to make the mould sides (the sides that are bolted together)  slightly flexible when the mould parts are unbolted so it can be flexed, popping the part out. This method isn't usually recommended by experts or books, but it has worked for me :-) As long as the sides don't converge this works pretty well for certain parts. It worked on the above part but only just!

Having said that, i recommend that you go with the traditional method as I don't want to be blamed if you can't get the part out! :-) The taper can be minimal. If the sides are of a convex profile then there isn't a problem.