I have found the answer to this but I'm posting it here in case others have the same issue and also asking if anyone has any more details ;-)
I use Firefox and Thundirbird's "Gnome Keyring Password Integration" Plugin by Guillermo Molina available here: https://addons.mozilla.org/en-US/firefox/addon/gnome-keyring-integration/
Everything has been working perfectly except that every time I start Firefox or Thunderbird I have to enter my keyring password, even if it's the same as my login password.
I could find nothing online about automatically loging in to keyrings other than the Default 'login' keyring
I was about to post a question here when I noticed the little Details drop down in the bottom of the password prompt. Looking here I found four options, the first of which was "Automatically unlock keychain at login" or something to this effect.
Setting this solved the problem, even if the keyring's password is not my login password.
Now if anyone can tell me how to change this back again now that it's set and the password prompt doesn't popup any more I'd be very grateful. :-)