Progress! Many thanks for getting me over that obstacle. Mind you, I'm still sort of flying blind...

Installed and ran Virtualdub. Looks bewilderingly complex to my novice eyes. But I opened the MPG and used Video|Filters|Add|Rotate. That gave me two apparently empty windows (Input and Output?) in top left corner of main window, with the latter being in the required vertical mode. Using File|Preview they both played simultaneously, and I was delighted to see that the right hand one was correctly proportioned.

I then used File|Save AVI, proceeded past the warning about large file sizes, and got an AVI file (without an extension, BTW; so I added '.AVI' myself. Is that normal?) D-clicking it in Explorer ran it OK

May I trouble you with a quick follow-up question? If I now want to 'merge' that 10 second AVI movie with another MPG of similar length (already in vertical mode, so need for rotation), what are the key steps please?

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Terry, West Sussex, UK