It sounds like rust fungus, though that is usually dark brown to orange, rather than gray. There are gray species of rust, but I have never heard of one attacking Hollyhocks. Anyone else with different experience?

I suspect that the pustules haven't started to produce their orange spores yet so just shows as grey (for now).

As Tug says, it will be rust. Unfortunately, any leaves showing signs will always have it now. You could try spraying them with a fungicide to stop further currently unaffected leaves getting it. It can weaken the plant and reduce the flowers developing to their full potential.

Thank you, Tugbrethil and Fractal. I feared it was more insect in origin as it looked like little raised bits, and had visions of the pest completely eradicating the plants. Not sure if rust is any better, but with a change in weather, maybe the plants have a bit of a chance to recover. I don't like spraying stuff if it's not absolutely necessary. But I'll keep an eye. Thanks again for your helpful comments both of you!