I think in the upper pic there are some bearings just visible. Like you say the design could be improved! Might even get away with a slim bearing race and some washers!
Good spot Ian. I had thought the yellow part to be one piece but I can see the bearings and races now you've pointed it out. You could improve it with a bearing but for the low rotational speed I'd be inclined to make a bushing that could be clamped tight (ensuring the wheel is held true to the rail) on the axle/bolt, that the wheel span on.
Lots of ways to improve it though if parts aren't available. If they are, careful installation might be all it requires to prevent another failure.
Go for broke, fit a proper roller door https://easyfitgaragedoor.co.uk/products/roller-garage-doors/ . Saves space and they just work, not silly money either. Used to have a DIY option to save a few bob.
Thanks for the other replies - thought I had "email me when someone replies" turned on but clearly not. Found a repair place in Croydon that popped over and sorted it today - 46mm wheel with a different bracket. He reckoned it was good for a few more years yet so I won't rush to replace it just yet.
At least I can get the Caterham out to get it repaired now!
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