Today in the same spot by the road in yellow box woodland I saw a male masked woodswallow and some dusky woodswallows. I got a good look at both so am pretty sure of the IDs. The interesting thing is that the masked woodswallow was much bigger than the dusky, while my field guide by Pizzey has the dusky only slightly smaller. The masked isn't on biby so I can't compare them there but it does list the dusky as only slightly smaller than the black-faced.
I think the dusky is smaller than the field guides indicate so would be interested to hear from people with other field guides or personal observations. Of course I couldn't measure the birds so it is only a subjective observation but there is no way I would mistake a dusky for a masked.
I think I know now why I was confused - the larger bird was actually a black-faced cuckoo-shrike, not a masked woodswallow. I have been seeing these birds around for years now and always assumed they were woodswallows, but woodswallows are much smaller. The name should have given me a clue. Also I never hear or read much about cuckoo-shrikes but they must be fairly common, I see them quite often in spring and summer.