Removed lots of these when I worked in bike shops here in U.S. It is the large diameter Maillard freewheel freewheel tool you'll need. In the U.S. any number of bike shops have this tool kickin' around. Not sure what the situation is in the U.K. but if you have any shops around, you might check to see if they've got one. The charge for pulling the rascal off shouldn't be expensive. The freewheel threads on, so you don't need the tool to put it back on. Also, it's probably french thread, so if you destroy it in the process of removing it with a pipe wrench per one of the earlier posts, it may be more difficult to find a replacement than the tool to remove it properly.