As a follow up I have been onto Apple support (which I initially thought this was having got here from www.apple.com/support, but that's another story).
They have informed me that I...
"... did not create the account with a free application, which is the only way to select the "none" option."
So, that's it, from the horses mouth. Despite Chris CAs assertions that our statements are all false it is the case that you cannot download free apps without entering your credit card details unless you specifically create a new account to do so.
Furthermore they declined to acknowledge my "feature request" for the "none" option to be added for existing customers.
There we have it then. Apple is, indeed, trying to force us to enter our credit card details. The apple id they created for me when I bought my mac cannot be used to download free apps unless I give them my credit card details.
Ho hum.