Just posting this in case anyone else has this problem - I searched through pages and pages of different Amphenol connections on-line, bought precision calipers, checked the size of the pins/sockets on an old Amphenol male/female I have - found that no modern connections appear to be the right size.
The calipers did it though. The Johnson connectors seem to be about 2mm - and I found 10 2mm banana plug male/female connectors plus heat shrink for £6 on e-bay - far cheaper than the Amphenol connectors from e.g. RS, once postage added.
https://www.ebay.co.uk/itm/281784986980
I’ll be making up the jumper wires this weekend and using in anger to diagnose my ignition problems.