The problem with mast bumping is that once in a low G situation and if the aircraft rolls to the right, the instinct for a low time PPL is to apply left cyclic to correct the roll, instead of first re-loading the disc. I have seen reports where the top of the mast has sheared off due excessive bumping, meaning the rotor head, complete with the MR blades, separates itself from the aircraft. If the 'breaking in two' witness statement is accurate (and if the fuselage is as complete as it would seem to be in the photo), then one could be excused for thinking that was the likely cause.
However, the witnesses also said they saw "a lot of smoke coming out of the back". Even an unreliable witness would struggle to imagine such obvious evidence, so catastrophic engine failure and subsequent failure to do an auto might be more likely. Whatever, I'm sure the reason will be obvious to the AAIB by now.