"I also like the idea that they may have been used not just for trade fairs, but for betrothal and 'marriage' ceremonies taking place between neighbouring groups (stopping the inbreeding and all that)."

There is the Teltown henge that was associated with the marriages <b>and</b> markets/fairs until 1830 (ish).

I also forgot about the henge at Tara. That's one mighty huge market for a supposedly scarcely populated period. This was preceeded by an enormous oval wood henge some 150-200m in diameter.

There are also four smaller henges scattered around the Boyne Valley and their closeness doesn't add up to markets unless each one specialised in different products.