You can get a hand-held Garmin Etrex (which can also be mounted) for under £100. Yes it's a GPS, but that said you can put down way-points, has speed setting, digital compass, etc. More robust than a mobile phone, in my opinion! Agree with Poly regards getting a compass though.
I'm not doubting you can use local charts, but I've seen fog banks roll in from no-where, especially at this time of year. Very disorientating and once you're in the pea soup it's like driving blind. Stick on the Etrex and select 'go to' to return to port.