Marley Greenhouse Spares

Hi ... my father has a Marley lean-to greenhouse ... aluminium & glass.


the centre door has come off the hanging / running rail, on the doors still affixed there are what looks like white nylon 'top hats' bolted to top of door frame.
On the 'loose' door, thee are only 2 T bolts sticking up, which I assume had the same white 'top hats'

i don't know if these are rollers or just a sliding plastic peg.

Anybody know where I can get Marley spares ? (and also if they are pegs or rollers)