Mitsubishi L200 starter motor not engaging

Totally out of the blue my pick up won't start, not even with a jump start from a van. It sounds like the starter motor is starting to engage but then just spins. I've charged the battery, which was a bit flat but is now charged and still no difference. Checked the ring gear on the flywheel (also O.K.). Off the vehicle the starter seems to run O.K. stripped it down tested, checked and cleaned everything but still the same problem? I'm totally stumped as to what to do next, but really need my truck back on the road as I'm a self-employed gardener.Would be very grateful for any ideas or advise.