As always, good advice. Thanks!Try the talcum powder trick, it may only be a pinhole and not leak till under presser then the engine heat may dry it before you can find it ...
Problem is I'd have to cover the whole (oily) engine more or less, as I'm not at all sure where it might even be coming from.
I don't usually fire the parts gun at a car, but I think I'll get a new cap as they can't cost much and they are a known failing part.
My new neighbour will be bringing his coolant testing kit with him when he comes back on Monday, so that should give me a yea or nay on there being a head gasket problem, in the meantime I will try to see if it is seeping through somewhere around the pump cover and/or the temp sender, as they are both above where I have seen some coolant gathering.
Have to confess I was shocked to find the coolant pump is "attached to and driven by the power steering pump" (Haynes) and is not to be seen behind the fan pulley. I am too old skool!