Do I need to reserve a Sat Nav in advance or can I ask at the desk when I collect the car?

Yes

I'd prepay gas and have told billed.

you can at he desk if u want too.

That will be an option when you pickup your rental along with other last minute items that you may have forgotten such as chargers car seats etc. No need to worry about a thing you're in good hands, you even have the option to pre pay for your gas at the counter. Ricardo G.