Yes, ziptest did the job perfectly - it showed a problem with U850 and U851 on Bank 0, and with U870 and U871 on Bank 2. I used contact cleaner and crimped the zip modules legs together so they griped the insides of the sockets rather than trying to push out into the pin-damaged sockets.
Banks 0 and 1 now work so I have 8MB. I think 870 and 871 require being replaced, sadly.