A collection car has a harder life electrically than a taxi. Its alarm, telematics and keyless entry stay awake around the clock, and two short drives a month do not replace what they use. Makers of these cars usually supply or recommend a battery conditioner that plugs into the mains for exactly this reason. If yours came with one, it is worth using.
When one of these cars will not wake up, the cause is nearly always a discharged battery and not a fault in the car. The technician brings power to it, starts it and tests the battery properly. A battery that has only been run down is recharged and kept. One that has gone flat several times and lost capacity is replaced with the correct type.