I found a study comparing the health benefits of standing desks and treadmill desks [here]. The overall conclusion is that standing desks provide some benefit, but treadmill desks provide a lot. It's a high-level review, which is good, but it's a few years old.1
A bigger problem than the study's age: it apparently considers only studies of electric treadmills. Electric walking treadmills that can fit under a desk cost between £300 and £1000, and there's no chance of persuading work to buy me one. Manual treadmills are significantly cheaper (and they can also be folded away when not in use).
Is there a study, or even better a study of studies, that reports the effects of a manual treadmill desk on health?