I have experienced on a couple of occasions, the operating handle coming out of the bolt/slide assembly during firing. It seems to snick in on its detent ok and it takes a tug to pull it out. I suspected that maybe the detent spring had broken or become weak and started to investigate. I removed the return plunger retaining pin and noticed that the slide block buffer was worn ( I have a new one thankfully) I could go no further as I could not work out how to remove the breech block plunger retaining ring so could not get at the spring, return plunger or detent ball. Having said all that the spring does not feel weak and is not 'scratchy' in its action as if broken. I added some penetrating oil and all seems smooth.
I thought about slightly reprofiling the operating handle detent notch, as the bobbin type that I am using is a little different to the stock item. As can be seen the bobbin is shallower and more rounded where the OE part is straight edged and deeper. Maybe I've answered my own question here, but in the last year or so this has never happened before and this last weekend it occurred twice in consecutive stages causing me to retire the gun from the days event. Not unsafe, but awkward to 'show clear' until the missing handle was recovered from the English mud! Meanwhile, next time at the range I shall try the original to see if the problem re-occurs
Any ideas or pointers gentlemen?

Try filing some material off the inside end of the handle. If that is a hair too long it can keep the detent from fully seating. You gould also try enlarging the notch slightly wider and deeper.

Thanks, will do

Followed your advice and indeed I have a much more solid engagement of the detent ball. As we are on semi lockdown over here at this time I can't field test right now, but while I was in the workshop I made up a competition Operating Handle from some scrap aluminium I had lying around. Its a nice snug fit and also engages the detent positively. These parts are not available for lefties and it gives me another option for when the shooting resumes. It is a comfortable shape to use, certainly cycling the action is easier than the OE handle and on a par with the bobbin type. We shall see how it performs in due course