Changing Super Administrator

You would have to be promoted to Super first and then you could delete or demote him.

If the original super is not contactable and you have access to the database, you could edit your user settings and change your level from 102 to 103. Copy all the user rights from the existing super and paste them into your rights.

Moreover, you can only demote the Primary superadmin through the database. The admin panel will not allow the no. 1 user to be edited under any circumstance.

Ok, so just to confirm .....
edit the database promoting myself to 103.
Go into PHPfusion to confirm this has worked.
edit the database demoting the original super admin to 102 ?

At 102 you will leave him as administrator. By making it 101 and emtpying the user rights files he will become an ordinary member. Don't forget to copy all rights under the previous SA to your rights field first.

I have done this and it seems to work ok apart from the fact that I still cant click on the edit button next to the old super administrator (user_id=1) to add or remove rights ....

You should do that in the database also. User_id 1 has a protected status in the admin panel.