OK I have found the DESIGN FEATURE that causes this problem...
It seems that in forum_topics, the toggle is not displayed if RSS Feed is switched on. I assume the intention here is that RSS should replace email notifications at forum level whereas email notifications are still allowed at topic level.
In forum_topics the code is:
If blnRss = False AND blnEmail AND blnLoggedInUserEmail AND intGroupID <> 2 Then
this can be replaced by the following to always allow the toggle (without disturbing the code too much!):
If blnRss = False or True AND blnEmail AND blnLoggedInUserEmail AND intGroupID <> 2 Then
BUT...is the design choice logical. First, I want email notifications at forum level AND I want RSS feed so why should I only be able to have one or the other.
ALSO...members with email notifications already switched on at forum level seem to have no way to switch them off again once RSS is activated.
Can we allow both please...?