Thank you for all the responses about the engine room.  I tried the plastic paint suggestion. Plasti Dip is available in the UK. It comes as a spray or brush on and there are the appropriate thinners and a primer for use on non-porous surfaces. It adheres well to the raw foam and 2-3 coats brushed on has given a tough surface. It is too early to say how well it will last. It certainly improves the appearance of the engine room, even just patching in.