Subsequent to the above, the problem returned immediately. I have a had a bit of time today to look into this some more. Precis of issue was one MTP device connected each time without lengthy rescanning but the other rescanned for 3 to 4 minutes every time it was connected.
What I found was that Calibre's mtp_devices.json file appeared incomplete and from an uninformed look at it, it seems maybe the swapping of a card in the problematical device caused that as it had two incomplete references to cards associated with that device.
I renamed the existing mtp_devices.json file to mtp_devices.json.bak, started Calibre, then connected (configuring their folders to be scanned) the devices in turn so that the .json file rebuilt.
The rebuilt mtp_devices.json looks sensible and contains about 6 times the information as the old; both devices now behave correctly when connected, hopefully forever
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So if one has this long rescanning problem, despite configuring the folders to be scanned, perhaps the first action is to delete the mtp_devices.json file and rebuild it as I have described.