One of the pre-war Army trials, I'd guess. Bike looks to be about a 1936 model with the straight gear lever, HF935 horn, narrow toolbox and 'scalloped' magdyno end plate.
The Scottish Service Dress tunic and the tartan trews were the pre-war working SD for the lowland regiments. It looks like 1908 pattern webbing as well.
It might also be that the helmet is still in the mid-1930s grey repaint scheme.