In reply to Nic:
I've always used moneymaxim for car hire excess insurance cover, never had an issue but never had to claim. Think you have to select the country(s) you're going to.
Obvs just be aware of all the usual things that catch some people out that you will need a credit card, in your name, that you made the booking with to put the excess hold on for whatever few hundred/thousand [local currency] that is.
My bro in law and loads of other people are caught out with that (he tried to use a debit card and they wouldn't let him, wouldn't let him use my card instead, etc.)