inline googlemaps plugin

I have a website (say) http://www.mainsite.co.uk and a mapped domain (say) http://www.mappedsite.co.uk.
When I access my site from http://www.mainsite.co.uk everything is perfect but when I access using the mapped domain http://www.mappedsite.co.uk then I get dialog box with the following:-
“This web site needs a different Google Maps API key. A new key can be generated at http://code.google.com/apis/maps/signup.html”.
Ok so I can appreciate the need for another key – my question is does anyone have any code that I can include the two different keys such that when the site is accessed the correct key can be used.
I am not a code person so fairly precise instructions would be appreciated.
Thanks