Success!!
Got them off... or rather my Dad did
Turns out there was only one hole, inserting anything didn't find any holes underneath... looks like a tool was needed to tighten the outer part onto an inner part to then engage the thread to unscrew... I think.
Basically, was impossible to do this without a tool as such, so my Dad 'squashed' the outer part with a mole grip enough to be able to grip the inner part and unscrew the whole lot.
Plastic standard dust caps going on now!!!