Thank you to all who have posted.
I have tried the various suggestions, but no success.
What does work is, if I load onto the Etrex as per normal, then transfer from the Etrex to My Collection in Basecamp, then transfer back, hey presto, they appear on the list, but a Faf!
However, Garmin have emailed with this:
Thank you for contacting Garmin Europe.
I will be happy to assist you with your enquiry. If this is the case then it would appear that the issue is with the files themselves. Can you please confirm if these were loaded as separate caches or as pocket queries? If possible can you please also provide the files in question so we can look into this further and offer additional assistance?
If there is anything else I can help you with then please let me know. Alternatively you can search for a solution here: http://www.garmin.co.uk/support
So I have sent the file.
I have also had a response from Groundspeak:
Hello!
We are happy to help you, but will need some additional information please. Please respond with the answers to the following questions.
What operating system are you using? (Windows, Mac or Linux)
a. Which version and service ? (Example: Windows 7, Service Pack 1)
What Browser are you using? (Chrome, Internet Explorer 7, 8, 9, Firefox 3.0, 3.15x, 4.0)
What URL are you having difficulty with?
Can you provide steps to duplicate this problem?
We look forward to hearing back from you, and hope to have answers for you soon.
I am determined to get an answer and shall keep posting.