Hi Congellous,
Do I coat them in epoxy laminating resin direct onto the foam ?
Yes, you should add a layer of epoxy and normally a layer of lightweight woven glass (we really like using 163g 2/2 Twill Light Weight Woven Glass for this purpose). The best type of epoxy would be an Epoxy Coating Resin - these cure with a harder, cleaner surface than many epoxies and so are easier to flat and polish to get a really good finish on your pattern prior to release coating and taking moulds.
I'm concerned that the patterns are very valuable and wonder if the precaution of taking a mould of these as they are, would PVA work on the foam as a release ?
PVA alone definately won't work as a release agent on the PU foam so I would strongly recommend that you don't do this. The best way you can protect your CNC machined pattern will be to coat it in epoxy and a thin layer of glass fibre to make it much tougher and more resiliant.
All the best,
Matt