Thanks, Ernst. It seems so obvious now. I exported the sheet to CSV and then imported using Memento into the Android. All of the entries arrived and I seem to be on the air again.
It is interesting to note that while I was looking around the library on the Android device I found the trash can which had all of my missing entries in it! So I probably could have saved myself some time if I had noticed that before.
Anyway, for the record, this is what I did:
Starting with:
- a google sheet with all entries in the library intact, field names in row 1.
- an empty library in Memento, but one which already has the fields defined.
Procedure:
- I exported the relevant sheet in the Google Doc to a CSV file on my PC (File/Download/Comma-separated Values)
- copied the file to a folder accessible from my android device (Dropbox)
- moved the CSV file from Dropbox to a storage location that Memento can access - Main Storage/Memento
- opened the empty library, found the Import and Export menu entry and imported the CSV file.
- the records all appeared in the Android Memento app.
Now, this is where it became hazy. I would have expected to be able to connect the library to the spreadsheet I had just exported from, but the only option I seemed to get is to create a new spreadsheet. I just went with the flow and let it do that. (Bear in mind that the new spreadsheet will probably have the same name as the one you exported from in the first place - In Google Docs it is quite possible to have two files with the same name - don't get confused!)
- identified which one resulted from the recent connection to Memento.
- added a record on my android device
- synchronized the database with Google.
Just a warning that had me confused for a while. Because the Memento UI has changed it took me a while to locate how to synchronize the database. Initially when I clicked on the ellipsis at the top right this menu opened:

I assumed that the

symbol on the second entry would do the job. When I clicked it however it shows the word "Synchronization" and then appears to be finished. Checking the spreadsheet on the web showed no change. Had me stumped for a while. The Sync function is in fact behind the ellipsis that displays on the Edit Library entry:

So we got there in the end. Many thanks for your help.
Ross