I agree, Ilangennith , so many series seem to end up like this. So many loose ends. I think it has been set up for a second series though, maybe where Matilda, with the help of Hal and Trudy, gets free of the Archangel or demon that is possessing her.
It was reasonably enjoyable, apart from the rush ending, but I will be annoyed if this gets a second series, when that other supernatural drama 'The Living and the Dead' (you know, set in Victorian times, with Colin Morgan aka Merlin) does not. TL&TD was SO much better, and the ending was pointing so clearly to a second series. Quite frustrating.
Perhaps the reason is that the Living and the Dead, being a costume drama, would cost more to make. But there have been so many dud BBC dramas, I wish they would make fewer, but of better quality.