Might it will be some helpful:
https://b2evolution.net/man/slug-delete

@joeest Thanks that link leeds me to another.
The first part says I cannot do what I want i.e. delete a canonical slug.
You should note that only user-defined slugs can be readily deleted.

By default, b2evolution generates two slugs when it first saves your post (the first one will be the canonical slug and the other will be the tiny slug). If you try to delete any of them, b2evolution will inform you that the slug is related to an existing element and that it cannot be deleted without first deleting its related objects.

but then it goes on to say look further

Important: It is possible to change the canonical slug of an Item by putting it as the first slug in the list of slugs in the Item’s edit expert form. This will allow you to delete the now non-canonical slug. For further instructions on how to perform this, please read: Editing Slugs On An Item.

Thanks, Not sure why I didn't find this, I often wonder why people don't check the manual properly, and that applies to me too.

So thanks again for the help and nudge. Will update here after using the info at https://b2evolution.net/man/editing-slugs-on-an-item

Ok it worked as described in the link https://b2evolution.net/man/editing-slugs-on-an-item

You may change the canonical slug for the item by adding the new one at the beginning of the field.
I moved the new slug that I had placed in order, to the front of the line, then when I go back to slug delete option, although the order there has not changed the option to delete has.

So not altogether simple but works fine.

I will add info to the manual.

Thanks again @joeest

Welcome @amoun
I'm glad it helped..