I've spent many hours, since yesterday, experimenting with the numerous options available in Avid and have managed to cure the problem by setting 'Compression Settings' to Photo-JPEG instead of DV-Pal and 'Quality' to Low.  If I increase 'Quality' to Medium, the jerky replay of the exported clip recurs.

Very many thanks to all who have contributed suggestions to this debate - Michael

I'll be surprised if that clip library is happy receiving low quality photo JPEG clips from you.  I'd want to get a clear specification of their needs from them, and then do my best to meet that spec.  But that's just me.

 Thank you very much for your input.  I have had "a clear specification of their needs from them" but have so far been unable to export from Avid, using their parameters, without getting the jerky playback.  I've just re-submitted the file to them at low compression and await their response.  The picture quality looks OK at my end.  I'll just have to keep trying.