I would suggest ''Maverick'', for phone GPS.
A few of us on here use it and it has definitely saved me buying an expensive GPS handset. (62s-£249)
Full Pro version is only £4, is simple to use and GPX files download straight from WH gps planner.
Plus I have used it over 2 days with 'gps on', on the same battery! (turned off in tent overnight)
This is quite something as even some of the best gps units will only last a day.
For grid reference I use the OS free version GR free version for SIX figure references. They both work fine and every time I have cross referenced them with others' units, they have been spot on.
Obviously, it goes without saying (and i'm sure others may mention it
) a map, compass and the ability to use them is by far the best and most reliable method.
Chris
PS. ''Our Cab'' may have some interesting methods that involve fish or similar!