Thanks for the idea Moth3r, although I think really I'm still better off leaving it as it is - if nothing else, it seems best philosophically

DE, I checked again in VirtualDubMod and am not seeing evidence of interlacing. Am I missing something?

Mystery solved - it's an mpv, as in Program. I only ever work with elementary streams, which (I'm guessing) lose the pulldown flag. I wasn't aware that VirtualDub would honour that pulldown flag and actually introduce interlaced frames.

Next time I'm looking for something to do, I'll take a stab at it.

DE