I can add nothing to the expert advice already posted here but would like to add my thoughts.
I always have my mobile with me but considering I need to leave it on the windowsill at home to try to get the intermittent signal, it is the last thing I would rely on. I have a Garmin that also loses the satellites in a valley or under a good canopy of trees. These things are great when they work and good fun but seriously a map and compass and the ability to use them in conjunction with the safety kit already mentioned is in my humble opinion a minimum requirement for anybody going off the beaten track.
Most people take the sensible precaution to have a map, spare wheel, spare lamps etc when driving and this is on our crowded roads. It never ceases to amaze me that the same people will then wander off into an area they don't know with no kit at all because it's leisure and therefore fun and "what could possibly go wrong".