Mileage was around 76K, so pretty low. Speaking with Russell and subsequently another ex MGR mechanic, I don't think the issue is as quite as rare as you may imagine, rather it simply goes undiagnosed, or mis-diagnosed. By no means common though.
As for the root cause, it's up for debate. Russell used to work for a main dealer some years back, and said the issue was quite common in low mileage 1.4 K series. One common factor was that the cars in question had all lead a very gentle life and been treated with "kid gloves" as they say - much as I treat mine (it rarely goes over 3K revs). Russells advice going forward was to let the engine work a bit every now and then. No driving like a loon, but just hold it in a low gear a little longer than normal once in a while to give it some excercise.
Interestingly, whilst I had a shed full of engine bits, I was finally able to check the pin holes in the cam covers. Both were perfectly clear - not sure why that should surprise me, but it did nonetheless
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Cliff