Bovington has two variants of the Mk V (Mk 5) on show, the original Mk V and the Mk V**.
The Mk V came into service in 1918 and saw action along side the White Russian forces during the Russian Civil War after a few successful actions on the Western Front, including the Battle of Hamel. It's main, significant, improvement was the introduction of a new engine system that featured an advanced gearing system that enabled one person to control the steering instead of the four required to drive the Mk IV, allowing the addition of two extra machineguns and a number of pistol ports around the chassis.
Looking at some of the detail shots, you can visibly see the influences for the archaic Leman Russ tank design from GW.
These last two are of a spare sponson from the inside. I'm assuming these are for the Mk V, but they were half way between the Mk V and another, later model.
Next up will be the Mk V**