Mark
Hope you're keeping well. Firstly I'd get yourself a copy of 'A Birdwatcher's Guide to Italy' by Luciano Ruggieri and Igor Festari, if you haven't done so already. It includes Sardinia sites as well as details on the subspecies that occur there.
I spent about a day there this Spring in a combined Sardinia/Corsica trip with Barbary Partridge fairly easy amongst other more typical Med fare - details can be found at
http://www.freewebs.com/richbonser/corsicaaprilmay2006.htm
Another trip report, written subsequent to my visit, can be found at
http://www.surfbirds.com/trip_report.php?id=967
You do also get Corsican Finch on Sardinia despite the name of this species. And, if you can, persuade the non birders to have a leisurely day on Corsica and you would be able to see the Nuthatch there.
Cheers
Rich