Hi Geoff,
Keep us posted regarding any costs.
The obvious place is on Lowgate next to Blue Bell Entry. If it is the market wall then the Council would have to give permission and might be persuaded to get involved anyway. I wouldn't even bother asking the landlord of the Blue Bell as it's Sam Smiths and anything to do with entertainment is no-go.

The cast plaque for Lal seemed to be green rather than blue. Would an official blue one be preferable in this case? The Watersons do feature in the famous Hull Music people pictured down the arcade at the album stall and the owner might have some input, perhaps even help to finance it.