Agree with most of the above. In my opinion, and speaking as an Electronics Engineer, the failure has been caused by arcing where the cable is screwed in, not the live contactor. Having now opened up the unit the actual contacts are fine with very little evidence of heat build up. Thus a loose securing screw in the contact is the likely culprit.
This again comes back to what I was saying initially, and as you point out, the caravan is now over 5 years old so it is likely the contact worked loose with vibration. I have owned the caravan from new so I positively know nothing has been abused, we can rule that out. I can't ever recall even having tripped the pole breaker actually.
But, coming back to my OP, in the same way that a domestic or commercial installation has to be serviced/inspected at certain intervals (OK, domestic ones are rarely, if ever, inspected) there necessarily must be some kind of schedule to perform an inspection where loose connections would be identified. Coachman refuse to publish this and simply defer to BCA (who make the board).
Furthermore, unless one is willing to strip out the offside bunk it is impossible to remove the consumer unit cover thus making inspection impossible. This is my second point which, again, Coachman won't be drawn on.
I get that components fail sometimes, this in't my point. Although the scary part is that the RCD unit still passes current. Even on a test bed it doesn't trip even if the neutral is fed back through a different circuit so the RCD part is very clearly defective. If there is any differential in current flow between live and neutral it should trip. The TEST button still works strangely but a proper RCD tester says it's faulty. Go figure.
Anyway, going back to my OP, 3 things.
1. Installation is untidy and 2 qualified electricians agree it would not meet the required regulations for a fixed installation. Since all Coachman 620/4 models (and probably others too) have this mess one can only conclude that Coachman are shipping out non-compliant units.
2. Installation does not provide for periodic inspection and is thus non-compliant.
3. The components used (NBSe breakers) "may" have an issue. Since mine failed to trip I was trying to highlight this to Coachman. This was never about the damned compensation culture we have these days, it was about bringing up a safety matter. Since neither Coachman or BCA are remotely interested you can draw your own conclusions from this. To be clear, I am not after compensation, I simply felt Coachman ought to notify their dealers about this. Maybe I'm deluded expecting them to do the right thing, after all it won't generate profit for them.
To answer a couple of questions further up the thread, Van was on a site, plugged into their pole which did not have individual 16a breakers for each socket. The van is rarely, if ever, run at the full 16 amps. We use gas for heating, 12v for TV and the only mains items are the fridge, a 1.5kw kettle and various phone chargers, oh and a crappy low power toaster which takes 5 minutes to cook 2 slices, so we tend to use the gas grill.
As for the installation now, NBSe RCD unit now replaced with a Hager unit (German) - pre-tested on a RCD tester, incoming cable from caravan inlet socket to mains RCD - I replaced the 2.5mm cable with 4mm to get a bit more bulk in the screw terminals, and routed it properly. Ends correctly crimped with ferrules to alleviate screws backing out with vibration. Front bunk modified so I can now get the cover off for periodic inspection.
Oh, and a large note; never another penny going to Coachman. They've had their last out of me. For a supposed "Premium" product it's pretty junk under the covers and their customer service is about the worst I've ever encountered. God knows what I'll buy next time but it definitely won't be a Coachman, that's for sure.