I realise we can't deduce anything about the Green Man from simply trying to imagine ourselves back into our ancestors shoes, but conversely I think we struggle to understand him because we don't have the outlook of them either

We do have the context however, which is the fabric of church architecture & the theological perspectives which they serve, otherwise they wouldn't be there in such profusion, nor would they be so stylistically consistent in their depiction of figurative human physiognomy however so stylised. The Southwell Minster misericord is pretty exceptional as a full-bodied GM, but it appears to be part of a narrative sequence made up of the images on the adjacent misericords - see Here for more. What that narrative is, isn't too clear, though in other tableaux we see the same figure liberating himself of the fronds, and other figures sprouting similar fronds from their backsides! It's worth bearing in mind here that misericord imagery operates on a similar level to the marginalia of (say) the Macclesfield Psalter (lots of green men there!) which at first glance would appear to represent something quite different to the text, but wouldn't be there without it.   

Are all foliate heads in English Churches pre-reformation then? Do they exist in Welsh, Scottish or Irish Churches? Do they exist in Protestant churches built after the reformation (or is there a continuing tradition of them being made for Catholic churches built since?)

Pretty much yes they are, certainly the significant ones anyway, which is to say an overwhelming majority that would indicate that there is a didactic purpose to such things, though the tradition of the image carries on to a lesser extent after the reformation. They certainly exist in Wales (indeed Lady Raglan's seminal thesis was inspired by those at Llangwm, in Monmouthshire), Ireland and Scotland (both St. Giles cathedral in Edinburgh and nearby Rosslyn Chapel are remarkably abundant). The post-reformation work on the exterior of Manchester Cathedral features a fair few Green Men, albeit styled on the pre-reformation ones found on the column capitals within; the 19th century bosses of Blackburn Cathedral are very fine too (though the real treasure there are the misericords from Whalley Abbey). The Roman Catholic Cathedral at Norwich (1910) is full of Green Men, many of which are on the column bases - see Here. One can't say for sure if the didactic tradition is embodied in these carvings or if their purpose is merely decorative / imitative of earlier imagery in the context of Gothic Revival - you certainly don't find them in other modern RC architecture, but in these carvings we find the same sense of everyman as we do in the medieval images, rather than the folkloric figure of the Green Man. Whatever the case these remarkably vivid carvings are certainly worth a look if only by of contrasting & comparing with those of the middle ages in the now Anglican cathedral.

IB have you been to se Lindow Man at Manchester Museum - I hope you left an offering if you did!

Manchester museum?? Never heard of it! The cultural dimension of our recent visits to Manchester have been taken up with the Holman Hunt at the gallery (all three Lights of the World, Isabella and more besides) but next time I'll be sure to check it out. What's acceptable as an offering these days?