Well I have been reading more of the forum and found a thread from someone who needed to replace a broken wheel on a bulldog, it seems you have to split the two halves to get to the centre cap, but even then the glue used to stick the two halves together has stuck the centre cap in firmly, the large wheels are fixed with the allen key fixing and the smaller idlers are just the push in fixing.
I think I might have to drill the centre or cut the top off (so it can be reused by gluing back on) to get to the allen key head.
