Well, it's quite an airy room, and they are all by the window which is usually wide open at the moment so they are getting light and breeze, but hmm, not sure if these fungus gnats are the ones 'cos you say they are black but these things are a little silvery and don't look big enough to even have wings. They are probably smaller than those red tiny spider mites you get on stone walls if that makes sense.

But perhaps you are right about overwatering - shall I let it go horribly dry for a while, I just don't know if I could do that!!!!
Or perhaps I should use a pesticide, even though I am dead against it usually.

Thanks for your help

Nima :O(