A car sat nav may work but is unlikely to be overly great to use, as it's designed to navigate by road and almost certainly won't have any detail of anything that isn't a road. You may also have issues with batteries (some are designed to be permanently powered) and weatherproofing. That said depending on just what you have it may work - I know a guy who cached with a car satnav before he bought a dedicated GPS.
Whereabouts are you? You might be able to find a local event where you can meet other cachers and talk to them about units, and maybe see if anyone has an old unit they'd be willing to part with.
Hi, thank you. I am in Guildford. Watching a garmin etrex 10 on ebay so fingers crossed