Here are two close-up views of the leather helmet issued to the first British tank crews, which may be useful to modellers; the first image is from Ernest Swinton's "Eyewitness" (1932), and the second is from a good article on British WWI tank uniforms by Paul Hannon "The British Tank Crew's War, 1916-18" in Military Illustrated No.40 (1991). This type of helmet was replaced by the Brodie shrapnel helmet in late 1916.
Ooops...that rather large first jpg got "crunched" almost beyond recognition - a good incentive to keep the file sizes down. I hope this re-sized image posts better.
This looks remarkably like the 19th Century helmet worn by the Sappers and Miners when working underground in siege conditions. One wonders if someone found an old store full of them and recycled them