I'm not very familiar with the battle of Edington, but I am rather skeptical about a purly testudo formation. It strikes me as though 9th cen shields would have been too small a diameter to adhere to a strict Romanesque testudo.
Being that it was made in '69, Anglo Saxon warfare tactics might not have been as well known or as thoroughly researched, so Roman tactics might have been improvised as a stop gap...

Still, with a cast like that, I'm going to look about and see if I can rent it ;-)