I'm 'quite' mechanically minded Lee..
A diesel ticking over when the engine is warm (AA in lay-by) no real discernible wear, would surely be better than one that's been thrashed from cold by a numpty on his way to work?
Actually that's what I used to think - not so sure now

It's not so much the wear, it's the fact that the oil does get changed according to the mileage, so the quality of the oil is breaking down, and does not have the lubricating qualities or should do, and in turn leads to wear.
With some of these cam chains the are a wet chain and use the engine oil as a lubricant, so if the oil has gone past it's best then it's will not be lubricating properly.
The other thing is even on tick over the chain is still in operation so it is still being used.
It may not be this that has caused the failure, it does seem like a common thing on the 2.0 vivaros for the chain to fail