Hi, many thanks.
I did realise after I posted that hidden menus might solve the issue, but only partially in my case.
In the URL manager the urls for the articles that are linked to in the SEBLOD list all include the homepage's menu ID. From many hours of google searches there appears to be an issue with multi language joomla sites and menu item IDs, which is the problem I have. Except I don't have a multi language website!
And I just cannot achieve the url pattern with a hidden menu.
Hidden menu has alias of:
/hiddenmenualias
My menu item for the SEBLOD list has url alias:
/whats-on
I want this pattern for my event articles:
/seblodmenialias/categoryalias/ID-alias
But what I get with the hidden menu activated is
hiddenmenualias/seblodmenialias/categoryalias/ID-alias
It seems to me that a SEBLOD list menu item does not assign the menu item ID to the article urls properly. It's possible that turning off SH404SEF will allow me to change the alias options in the SEBLOD content type and list in order to achieve what I need, but I'm afraid to switch off SH404SEF because I've inherited a large website, and I don't want to lose any of the urls that already exist in the system.