Looking at the BBC site says there are five episodes, but has no date when we might expect the next three.
I think they're worried because it's that good (9.6 user rating on IMDB), it's being wasted going up against The Syndicate. And there's already "24hrs in Police Custody" on Mondays, and from next week, "Bent Coppers Crossing The Line" is on BBC2 Wed, both at 9pm. It's very crowded. This does deserve to get as wide an audience as possible. I wouldn't be suprised to see it turn up on BBC 1 after some of these finish.