I had a quick search regarding this but the only articles I found hark back to 2015 when many hybrid busses were running mainly on diesel due to poor battery performance.I saw a few months ago that about half of these London hybrid buses are off the road because the battery at 5 years old has expired and it costs too much to replace them.
I wonder who forgot to tell 'em?
'Hybrid' Routemasters reliant on diesel due to battery issues
Major problems reported with London's new hybrid Routemaster buses, particularly with regards to battery life, has led to TfL being forced to replace batteries in 200 of the vehicles and defend their green credentials
www.wired.co.uk