Update:
I should have thought of this one earlier, but if I remove the rewrite section from the web.config file I can get back into the site.
I thought that getting here would allow me to solve the problem because I’d be able to select the ‘Default’ url setting from the rewrite options & everything would be back the way it was.
What is happening though is that when I do select the default ( https://66.228.122.48/blog/?p=123 ) and save the choice I’m locked out again (as in EVERY page gives ‘500 – Internal server error’ ).
Before I set the custom string, although choosing anything but the default resulted in an error viewing the posts, the admin side of it always worked & I could undo my changes with no problem.
Does anyone have any idea what’s gone wrong please?
Many thanks.