I would take that with a massive pinch of salt. Ride comfort is affected by many factors in addition to weight, such as suspension design and where you happen to be sat in the coach. Just looking at weight, it matters how that weight is distributed between the body / bogie, how much is sprung or un-sprung mass, and where the centre of gravity is. This isn't to say that the Mark 5s will have better ride comfort, just that I will wait until I've travelled on one before making any judgements.
Also, the non-sleeper Mark 3s weigh between 33t and 38t in tare (no passengers), depending on what layout / type, so I don't know how accurate the 43t figure is quoted above. The Porterbrook Mark 3
brochure quotes 9.375t axle load in tare, so ~37.5t per coach (ETA the 37.5t is for normal passenger layouts; I didn't find a weight for a Mark 3 sleeper coach during my very brief Google).