The demould was easy, but I did have to hold the part in the mould to remove some of the peel-ply in strips, as the part had freed itself before all of it was stripped away. Not certain if some movement during demould could have caused the defects but ift feels odd that the damage has only followed the lines of the fabric matrix if so?

In the meantime, I'm pretty sure you are talking about what I call crazing, which is hairline cracks in the gelcoat? This is always due to stress and movement, but can be caused by a number of things. I think ajb100 is on the same wavelength as me here, as this sounds like a potential mould sticking issue. The one time I got crazing in my in-mould coating was because I had applied a bit too much pressure buffing off the residue of my release agent, buffing off a lot of the release agent. So when I demoulded, the part didn't stick bad enough to destroy the mould, but the coating on the part was very badly cracked from the stress of demoulding. 

Alternatively, is it happening in exactly the same spot each time?

A couple of recently infused parts have demoulded with small dry areas of crazing on the part surface. I'd sprayed GC50 into the mould first, left overnight and infused the next morning.In both cases full vac was achieved and drop-tested for 30 mins before infusion