But if they are the existing contractor, they will have diesel buses. They may start to get to be too old (not sure if TfL have an age limit), but reliability could start to reduce with age. One thing that I have found strange recently, is that London bus companies are selling TfL compliant buses to other London operators - in the past, they would just send them to other parts of the country. Thus there could be a supply of existing diesel buses.
Having said that, I'm not sure how much supply there is of smaller single decks - for example, when the Roundabout routes in Orpington come up for tender renewal in 2 years' time, will there be any non-time expired diesel buses available. The existing fleet is 2010 - 2012 mostly.
Conversely, are there any suitable small 9.0m electric buses? ADL's smallest E200 EV is 9.6m long. Are the Mellor Strata robust enough?