I know this thread is now a year old but just wanted to add my 2 cents:
One of my fave versions is Tony Capstick's version (from the Live At The Crypt album) which I am sure is in G (Tony seems to have favoured the key of G quite a lot) He also sings Mike Harding's song "The Wild Geese" starting on G too. It was hearing his versions that made me want to give them a try but when I try & sing them I find that with both songs they work more easily for me in C or D but I still kinda wish that I could sing them in G as Tony does as it's those versions that drew me to the songs in the first place. But anyway, great songs both! Also very well observed point by Pete N ... I hadn't noticed until I read it above but the melody to "Sweet Thames Flow Softly" is almost identical ... certainly very similar ... & another song that I've really fancied playing for a while ... so based on that ... two for the price of one really! & both really great songs!