Noel, You could well be correct about the mounts. I thought that I had seen other pictures with a lower mount but now suspect that they might simply have the bipod hooked over the hand rail.
This is a variation designed to hold off the umbrella-wielding hordes
I'm not sure if Sidecars with mounted Brens were actually used apart from training (and propoganda purposes).
I haven't seen any photographic evidence of Outfits with the BEF in France being equipped with fitted Brens and most captioned pictures show Canadian units training in the UK in 1941 / 42. The Norton advert dates from April 1941 and carries Southern Command markings.
I wonder if any film exists of live firing with the mortar. Not the most stable platform, I'd have thought.
Rich