Hats of to Handy' and Spin', it is indeed a humidistat and made by a Leicester company Mellor Bromley, after speaking to one of their guys today they started making these in 1937 and were often found in the textile industry especially yorkshire, I'm trying to get some more info off them so will add to thread if I do but I would say looking at the electrics in the mill (VIR in conduit) that this may be approx 70 - 80yrs old.
A massive difference to modern tech' where you can now get pcb mounted sensors that do the same job.