Dave, I had exactly the same problem recently. As GRB says check your battery more closely. Kiwidave gave me a very good tip to check that the battery is holding the charge under load-
put your voltage meter on the battery with everything off and then switch on the ignition and lights, now see what happens to the reading. My battery was reading a healthy 12.75 V with the ignition off but when switched on the reading quickly dropped off a cliff.
Just because the bike will start off a jump in no way proves that the battery is OK but it does indicate that your starting system is working when connected to a healthy battery.
Because I was only checking the battery voltage when the ignition was off I wasted a lot of time trying to find the problem elsewhere
