The bus at the back is definitely an "R" class - the "flangs" at the side of the radiator are a dead giveaway! Furthermore, if you look closely, the rear dome appears to be the squared off and upright type of the earlier 1938 vehicles - and the block-type vents.

I think the vehicle is a TD4 and one of the R1 - 50 batch.

The post war OPD1s had the later Leyland wider radiator with no side "flangs" and the continuous strip vents above the lower deck - I think.

Sean