LDV Pilot (2000 X Reg) randomly wont start?

At random points, my van will not start at all. No clicking from the ignition or engine when i turn the key. Just nothing. The fan and stereo come on, the lights work fine. If I bump start the van it will start, if i try and move the van in gear then start it, it sometimes works. This has happened to me twice now. The garage said it checked my glow plugs, battery, alternator and starter motor, and all is fine. The annoying thing was, is that it didn't give the garage any problems when it was there. I'm putting the problem down to the starter motor, but need some advice before i purchase a new one... as ive read on this forum that it may not actually be that. any suggestions?