This is my first year growing tomatoes in my unheated greenhouse. I'm using growpots and I wondering how much watering I should apply should the compost be kept constantly moist or should the the surface of the compost be allowed to dry out between waterings.
Peter, better to let the mix dry out between waterings, by which I mean dry out pretty completely. Very very few plants like damp feet and toms aren't one of the very very few. How big are your pots? Poke a finger down deep into the mix to test for moisture. If it's dry down to the depth of your finger - subject to the size of the pot - water.
As I've posted elsewhere here, toms are very sturdy critters that will produce much better for you if they're treated with - for want of a better term - controlled neglect. Pampered toms - over-watered and over-fed - will never achieve their maximum potential.
As an example, I grow them outdoors here in Tuscany. The summer temps can sit in the 40s for weeks on end. In those conditions, I water infrequently - about once a week at most - but very deeply. They produce tons of fruit.
Now obviously toms in pots have different requirements simply for the very contained space in which the roots live. They can't burrow down in search of moisture. But the same general rule of thumb applies - make their lives too easy and comfy and they won't give of their best.