I noticed that in the tutorial on mould making for a pre preg air box that the mould was laid up by hand with no diffusion or vacuum. Was this sufficient to get a defect free surface due to the gel coat being used?

Great videos ...

They do definitely use vacuum? 

https://www.easycomposites.co.uk/vacuum-bagging-sealant-tape/how-to-make-complex-split-multi-part-mould-for-composites

This mould is the unimould system with chop strand etc. No vacuum.

Sorry, misread the post and misunderstood the question - I thought the question was about the pre-preg video. 

So yes, there is no need for vacuum when making a mould; wet-lay works great if you use the correct materials.