Thanks to everyone who has given their time to help me out. I've now tried different adapters and found that the inexpensive iJiGui from Amazon works whichever way-up it is inserted. So here is a full recap of my findings:

1. Others have suggested elsewhere that it appears to be necessary to enter Settings - Privacy & Security. Scroll down to Security - Allow accessories to connect and select Always. [I did that first. I'm now not sure if it has helped since I've not yet tried restoring it to default.]
2. Neither of the two types of Syntech USB C to USB OTG (including Cable) Adapters that I was trying (before posting) worked - whichever way-up they were inserted.
3. I had already in my possession a pack of 3 adapters (unfortunately origin unknown). All of them worked BUT only when inserted one way up!
4. I've since bought and tried the two pack iJiGui adapter from Amazon and they both work - whichever way-up they are inserted.

Conclusion: There are some simpler adapters (presumably having no active components within them) that will work with the older legacy keyboard. The Syntech adapters (that do have active components within them) are designed to work with later USB 3.1 [I think] standards and speeds. AND There may be also something in-between! SO this connector compatibility issue appears to have become a minefield for the unwary when connecting old to new. As they say: "Your mileage may vary." You may need to study the whole USB connector topic in detail depending on your need.

Hope that helps everyone. Thanks again to the respondents.