.... all hybrids cut out the engine on idling and leave the electric motor running and when you move off it kicks in.
....actually that is not correct. The Volvo hybrids do this, but the Dennis Enviro 400H does not. The diesel engine is always running, even on idle. Generally speaking there are 2 types of hybrid: ones like the Volvo (and also cars like the Toyota Prius) which mix a diesel powered traction, with direct electrical traction. (So in those types, like the Volvo), the diesel engine will cut out while the electric is giving traction, or during idle.
But the 2nd type of Hybrid are those where the traction is always coming from the electric battery, and there is no direct drive from the diesel engine to the axle. (similar to a diesel electric locomotive). In these types, the battery will power the bus but charge is regularly needed so the diesel engine revs to charge the battery. But the diesel engine does not 'cut out' on these types. When it is not 'revving' to charge the battery, it idles. This is how the Dennis E400H works.