Well after spending 20 mins of so beating the life out of the valve the wrong way around (clockwise to loosen)...
I had a look at the edge of the threads that were still exposed and discovered that they were infact a normal (anticlockwise to loosen) thread...
It came of really easy after that, one heavy thump with a wooden mallet on my wrench and the rest was done easily by hand...
What I wasn't prepared for was the amount of oily residue that came out of it when I filled it with water... Made a mess of my decking, but no serious drama, a bit of warm soapy water cleared that up easily before SWMBO got home...
Now to work out what to do with the now empty and safe container...
I like the looks of the one above, is the side part of top plate made from an old steel car wheel...?
I'm sure I've seen instructions, or other pictures of how to make one like the really cool one above, but for the life of me I can't find them...