Thanks all,
I really appreciate all your contributions and tips, the truth is I've had grief with the system from day one, in an attempt to resolve the problem I started by studying the cable and Ford Explorer door release lever setup, I found my cable was partially seized so I've completely eliminated this part of the system by removing it for the time being.
To replace it and operate the the sliding plate pin receivers (the catches) independently, I've attached a separate run of nylon paracord to each catch, I wasn't impressed with how MMC attempted to operate both catches simultaneously from one cable so I chose to fit two completely separate cords to conclusively prove the issue wasn't down to the cable adjustment between the two catches.
My two paracords exit through the offside seat belt aperture in the carpeted rear bulkhead, I then put a mower inertia pull start handle on each paracord to give me really good purchase when operating the sliding plate pin receivers (the catches), this is all a temporary measure designed to completely remove the MMC cable and Ford Explorer lever arrangement from my list of suspects.
To be honest my temporary Heath Robinson system works perfectly and in my opinion isn't much more crude that what the MMC gave us, my paracord pulls also conclusively prove there's absolutely no issue whatsoever with my offside pin receiver and pin adjustment, my temporary arrangement does what I wanted it to do by proving without any doubt I only need to focus on the problematic hood latching on the nearside.
My next step was to carefully study the relationship between the nearside pin and it's sliding plate receiver (the catch), after removing the spare wheel I found the sliding plate catch is secured under the rear deck with two allen head screws, and following this discovery I set about adjusting the sliding plate receiver trying a number of different positions, I tried many different pin depths by winding the pin in and out of the captive thread in the hood frame, I also noted gently bending the pin can help a little but this is a horrible way to adjust something so I'm trying to do this as little as possible.
Finally I disassembled my nearside pin and reinstalled it without its compression spring to see if that helped, it didn't, once again the pin either failed to latch securely enough to hold the hood under tension without popping out, or with a different set of adjustments it hooks in so well I can't release the damn thing, this proved to me the pin spring isn't the issue here.
Sadly, no matter what I do, no matter what combination of pin depths and sliding plate catch adjustments I try, my one problematic catch on the nearside either refuses to latch to the pin sufficiently well enough to hold the hood under tension, or it latches so securely I can't release the damn thing, after what feels like hours of tweaking and adjusting it seems arriving at the perfect alignment and balance between these two fail states is proving impossible to find.
Now with all that out of the way, I'd like to clarify a couple of your comments:
1.
There is a slotted part on the hood frame, secured by allen head 6 mm bolts, that can be adjusted to move the hood back and forth. If they become loose, the hood can move and then the sprung pin does not centralise as it should.
Excuse my ignorance Deejay but I'm trying to understand this better, as outlined above I've adjusted the sliding plate receiver (the catch) that's secured under the rear deck with two allen head screws, but I haven't as yet found the adjustment on the hood frame you're describing, any chance of a photo?
2.
If the hood tension is inadequate when locked down at rear, the hood material to front rail tension can be increased by loosening lock nuts each side and turning complete catch one full turn such that it still accurately engages with windscreen side catch.
Actually Richard I feel less tension on my hood would help as there seems to be excessive load on my nearside catch, when I adjust it so it does latch correctly the excessive tension immediately pops it out, so less tension would definitely help. As this only happens on the nearside I'm wondering if my hood frame is twisted? However, just to clarify, are you saying the best way to achieve a change in hood tension and load on my problematic rear nearside hood catch is by adjusting my nearside header rail catch up front?
I'm going to have one last crack at it today, but if I fail to achieve success I'm thinking it's likely that my nearside rear hood catch has become irreversibly worn so will require replacement? I remember this type of bonnet catch on MGA;s I've worked on but now we know they were also used in Minis I assume new ones are available from the usual suspects like 'Mini Spares' etc?
Thanks again everyone, Dave.