Cheers guys! Its more expensive to rent one through the carhire (~$90) co than it is to buy the map (£55) for my tomtom Hopefully i can find someone who is a bit more reasonable.....
When we went, if you hired the satnav they ley you have free child seats which we would otherwise have had to pay $60 to hire so it made sense. The Garmin they use is very good though.