I don`t know if anyone has mentioned this, but there are some excellent videos at the moment on " You Tube" (
www.youtube.com) .Just punch in Cromwell Tank in the search box, and you will see "Cromwell Tank 1/5" , ..2/5, ...3/5 etc. It is a mini-documentary of this type covering the Centaur, Cromwell and Comet, I think done by the Bovington museum possibly. There are also documentaries on the "Sherman Tank", "Churchill Tank" ( British infantry tank history), and "M3 Lee Tank" similarly done in 5 sections.Each one has historical film, interviews and current film of them. Really well done, and it`s free