Thanks guys,
I have just measured my original one with a vernier caliper albeit a not very good one and get 24mm so I'm guessing it's a H24 that is needed that would atleast make sense.
The problem is living in Mallorca and ordering stuff in UK is a pain in the ass If I make a mistake. I run a front brake and want a top cap with extra opening for brake cable and there really isn't a great selection out there and I quite fancy the Odyssey one but of course it's also the most expensive bugger out there, anyway unless anybody knows any better I recon I'll just go ahead and order it and hope my measurment was correct.