Thanks Ian.
This is what happens when they stop going round:
You can certainly tell the two which were still rotating.
From the witness marks on the pins, it would appear that the rollers were stuck on the pins, rather than there being debris around them. There was a certain amount of grass but it pulled out easily and wasn't jamming any of the mechanism.
Fortunately, the engineering department

chose a soft grade of aluminium for the rollers, making them sacrificial, thus saving the more expensive track links from excessive wear.
Something else to add to the spare parts order Gill
Adrian.