Chug,
We'll have to differ on this one, but I can only reiterate that lumping them together as having 'common failures' is too simplistic.
I do not say that problem was a factor in the Mull accident
It is really a red herring though - the status of the MAR was an on-going issue which would have been very damning if it had had a bearing on the incident - but it didn't. I propose not to get into the debate here though - its too emotive - but by the same token that you can argue with me that it could be an issue, it is possible (& easier) for me to argue it wasn't. Suffice to say you can't draw parallels to what happened on Nimrod, and on balance of probabilities, Mull was 99.9% not a airworthiness issue.
For Nimrod it is clear that a design flaw which had a direct bearing on its safety during normal operations persisted in the in-service aircraft - no matter how you dress this up it is 100% an airworthiness issue. The MoD conducted a Safety Case exercise (which as discussed above was not even legally required, but did/does represent best practice) but which still failed to flag up the problem - the flaw goes back along way, probably to the days before the term Safety Case was even dreamt up (anyone know for sure when this term became common, late 80s/early 90s?). There are lessons to learn here, I've never questioned that - but from what I can see they are not necessarily obvious ones.
Hercules is a bit different in that its a capability/threat thing. To draw a simplistic extreme, you could find yourself shot down in combat by a Sukhoi Su-27 while flying your Tornado F-3, or even a Typhoon. In this case you could argue the MoD had been negligent by not sufficiently reducing the mission/combat risk to ALARP - it could have equipped the RAF with F-22s which is a bit of technology that is (perhaps only slightly) more likely to have shot down the Su-27 before being shot down itself. Should ALARP/fitness for purpose/airworthiness apply here to these type of risks? Training and tactics probably play a bigger part than the technology in the outcomes here anyway so where best to invest? You could debate this all day but I hope you can see this is not the same thing at all as an aircraft that may catch fire/expode all on its on during normal operation.