I think that the fuel was still coming out of the right carb because the left was also over filling and the fuel was being transferred from one to the other through the breather hose. I took the float bowls off as suggested by Ladder106 and held the tree on using a large elastic band. I used the old float valve bodies and soldered up the hole in each of them, fitted them to the carbs and - no fuel. Fitted the new float valve bodies and put a finger over each of them and again, when I switched on the fuel - no fuel. Took a finger off and fuel flowed (under quite a surprising amount of pressure). It was then that I noticed that the new float valves and their bodies were a slightly different length to the (what I guess were OE) old ones that I had replaced. I therefore added 2mm to the float height and tested again - dry carbs
I have now refitted the carbs and have run the bike on the bench and everything is good and dry.
Thanks very much Ladder106 for talking me though this - it's great to have someone to be able to bounce stuff off rather than just sitting in the cave not really knowing what to try next