I believe a form London Transport Routemaster is now used on this Christmas day service, but as other historic London bus enthusiasts will know, this annual service was originally operated by a preserved former London Transport 'RT' type bus. Both are very similar - dating back to a time when London's buses were purpose-designed by London Transport's own engineers to meet the needs of London, mostly with AEC engineering and mechanics and Park Royal body work, both firms in Chiswick. They also had very similar AEC 9.5 litre engines. The RTs preceded the Routemasters, 150 being built from 1939 until the outbreak of WW2 and the remainder - several thousand, the RT was the largest standardised bus fleet ever in London, between 1947 and 1953, and was last officially in service in 1979. The Routemaster fleet was built between 1956 and 1967 - some two and a half thousand were built for London, they survived officially in service until 2005. Just thought I should mention, because this Christmas Day service has in the past used both the RT and Routemaster types!
David Beeney ● 9d