Me and Roy did go out on Weds to have another look at the lid - basically the tension between the two nuts wasn't sufficient to maintain the latching process without something or other coming undone, or being too tight. Ideally a spring washer in the system would help. As we brought kit for a M16 problem and then found we had a M10 problem (!), we were somewhat ill-equipped to do the dream job we planned. However, Roy's magic toolbox came to the rescue, and so we drilled through the two captive nuts and the bolts, and inserted tiny roll pins, locking the nuts at the appropriate tension. These were then snapped off. After some tweaking with me inside the lid and Roy outside, we managed to bend and bang away at everything until I could easily latch and unlatch it with my fingers from inside, but it definitely needs a spanner to open it or properly lock it up from outside.
At some later date, now he's got all the bits, I'm certain Roy will probably want to go back and redo all this at M16, and who am I to stop him? I may go along for the ride if I'm free - but for now, it's secure