It was the Covenanter tank. It was a cruiser tank, I can't remember whether it was the IV or the V. (CL may have the answer already)
They were built by a railway engineering firm, and Britain was so desperate for tanks after Dunkirk that they couldn't afford the lost time needed to shut down the factory and retool for a different model, so they just kept building them. I think that the most bizzare feature of the Covenanter is that the radiator for the engine ended up being bolted to the front of the tank, as there was no room at the back.