After selecting gear down, a safe indication could not be obtained for the left main gear. The pilot diverted to a more suitable aerodrome, where further attempts to lower that gear also failed. The gear was retracted and a wheels-up landing carried out. The pilot subsequently advised that a similar left gear unsafe indication had occurred 3 landings prior to this particular occurrence, but on that occasion after recycling the gear a normal extension was obtained. The left gear had not been rigged or lubricated in accordance with maintenance instructions, and the left gear uplock roller had seized. A check that the rollers are free to rotate is part of the normal pre-flight inspection. Indications were that neither of the maingear rollers had rotated for some time. There was some doubt that the pilot had been advised of the gear roller check requirement when he was undergoing endorsement training on the aircraft.