A quick update. Eventually I gave in, and took the bike to my local bike shop who did a great job of removing and cleaning up the EBB. I had a chat with the mechanic afterwards who said that the EBB had been put in the wrong way around (which he sorted). Also that the EBB consists of two wedges, and the trick, once one wedge has been loosened, is to give the wedge bolt a tap, on threaded side, to loosen the other wedge. That was the trick I was missing. After that he said the EBB rotated freely.
Thanks all for you suggestions, great forum!