Thought I'd see if such a thing as a clockwork Great War tank exists and lo and behold there's one on ebay. Made by Marx and about 22cm long so I am not what sort of scale that is. A bit expensive for me but I thought maybe some of you might like a look.
Checking it out further there's quite a few of these from different manufacturers but obviously they're very collectable. I suppose what we should wish for is that someone does a modern repro.