Basic royal mail problem

I am having problems whilst editing the royal mail air mail shipping method. When I change the postage rates and update, the edit and remove buttons vanish and I am unable to do anything. I have deleted all files for this method and then inserted them again which brings everything back but when I edit again the buttons vanish again. Never seen anything like this???

Beats the heck out of me! That usually means there's an extra blank line been added to the php file during an edit, but you're not editing the file.
Does it happen if you don't change anything but merely edit and then hit the update button?
And secondly can you be a bit more explicit on EXACTLY what is being changed. Perhaps the field length is being exceeded?
I've just an edit on a system , changing rates, without any problem, so it doesn't seem to be 'generic'

To expand a bit more, the Shipping 'Edit' - 'Update' code in Admin is part of the core Zen Cart code, not part of the addon shipping module. The addon module contains install/remove routines which are called by Admin, but everything else is core code used across all the shipping modules.

We recently installed several of the BASIC RM modules and one of them was Airmail... and we are not experiencing these issues. We are still on ZC 1.3.9h with the site in question.