Many thanks to 2Bold2GetOld for the prompt response and great photos!
Now I can see that a PO has already removed my lower roller, but as he also fitted the much-sought after plastic frame guards, they obscured the mounting point. Everything seems to work fine without either roller now, so I think I'll just stick with that for now.
While I'm here, I just thought I'd show you the problems we have off-roading here in the UK.
Never mind dogs and shotguns - here they keep you out with battle tanks!