Hi jimbo, if its any consolation i am also missing a idler cap off mine
I think the sprue is available from axels for about £15 plus p&p, i took an existing cap off on mine and made a
mould with blu tac, filled it with car body filler, its a passable replacement but not perfect.
Dave (forgebear) has walker bulldog wheel caps in that could possibly work with modification.
regards john
