Hi Anthony. Since my post a friend of mine has printed a chest box for me. Now looking for the tabard with white cover to buy or make. Don't know what the brown backing is made from . Thanks. Mancuniancomman.
It's actually a dark gray material, but it appears from photos to have a mottled appearance with small strands of various colors visible in closeup photos. This has led to speculation that the material used was something like emergency thermal blankets, made from recycled textiles, but I don't believe that's been confirmed.
Not sure about availability in the UK or if the terminology may differ, but in the US the best match is likely a fleece or felt, similar to this: