Auxilier Bob Millard adds;
A wild guess. The top of a field kitchen unit, the cook pot goes in the hole, the fire vents through the slots on the top? Bob
Bill Ashby has been in touch with the Tank Museum at Bovingdon and they say the following
I can tell you what I think it is; the front engine cover plate for a Medium Mark I tank of about 1923. See image below.
However it depends on a number of things. The round thing (if I am right) is the air inlet for the air-cooled Armstrong-Siddeley engine. It contained a fourteen inch diameter fan so if the hole in your plate has an internal diameter that is something over fourteen inches then that would be a good clue. I am a bit puzzled by the location of the triangular grill. There are very similar grills on these tanks but not in that position, relative to the hole.
If it is not a medium tank then it could be an artillery tractor, known as a Dragon but that would rather depend upon where it was found – whether the location had more to do with tanks or the Royal Artillery.
I have also been looking of Flickr and found these two images which appear to show a similar panel. Were these on a later model?