Thanks everyone for your comments and suggestions on my popping center latch. I suspect, as Tom indicates, that the geometry of my top is not quite right. Since the top is not original (it is canvas, not the vinyl that would have been fitted originally),and not a great fit, the problem may be beyond simple adjustment. The pin of the center latch doesn’t fully seat in the hole in the windshield support because the front seal prevents if closing any further. That said, the further forward I pull the top, the more the latching problem is exacerbated. Today, I slightly increased the shimming of the front pair of screws by adding a further washer between the latch at the top, and as per @L.Lynn 's suggestion, drilled a hole through the latch assembly for a hitch pin.
Apologies for the glare - I shouldn’t have polished the chrome…
The combination of fixes means that the latch now holds without the hitch pin when stationary, and the hitch pin should keep it secure when on the move at speed. I found a brand new replacement center latch in my collection of parts, so drilling the original was an easy decision to make.