You'll have fun Jon, you can't usually get at the circlip because the lever blade gets in the way, and you can't get that out easily. Undo the lever pivot bolt and take it out, and try to get hold of the push rod with a pair of long nose pliers, you'll have to squeeze very hard, and use the lever blade to unscrew the brass barrel from the push rod. Now you should hopefully have the lever body with the push rod poking out of it, so you can remove the circlip and the pushrod should come out. If you leave the topcap sealed, sometimes you can manage to keep the piston inside the lever, normally it pops out though, so once you've fitted the new push rod and put the circlip back in, you'll need to rebleed it. It's a very fiddly job, but I've done it a couple of times and never struggled too much. I put a big jubilee clip round the pliers the last time, it was just slipping all the time. Too quick for me, how did you get around the lever blade to get the circlip out? Mabe it's just my plier/point thingies, they have massive arms on them.