Middle episodes of three-part dramas can often feel like filler, but the tension held up pretty well in last night's Place of Execution, where Catherine set off for Scardale to try to catch up with George Bennett in the present day, while the 60s version of DI Bennett was closing in on his chief suspect for the murder of young Alison Carter. Or was he?

While Philip Hawkin was undergoing the kind of round-about questioning that only a suspect against whom there's no real evidence is subjected to, some real evidence turned up back home. His wife found a blood-stained shirt behind a radiator. So tightly packed behind a radiator, it has to be said, that you wonder how she ever found it, or was able to recognise it as a shirt.

Hot-footing it over to the manor, DI Bennett and his trusty sidekick Tommy retrieve the shirt and discover it's wrapped around a gun. Exactly the same kind of gun they were looking for. The kind of gun Hawkin had denied knowing anything about, even though it's wrapped in one of his shirts. You don't have to be a crime drama fanatic to know what comes next: warrant; search.

While Tommy searches the rest of the house, DI Bennett concentrates on the darkroom. We know it's a darkroom, because it's lit with a red light. This took me right out of the moment. Darkrooms are always equipped with both safelight and normal, white light. Otherwise you can't see what you're doing once the film is safe. And if you're conducting a search, well, you'd want it as bright as possible, surely. Except in dramas, where directors always assume the audience needs to see a red light to get the message that we're in a darkroom.

It did add something to the atmosphere though, I admit. And the slow unravelling of finding the concealed safe, finding the key to the concealed safe, opening the concealed safe, finding an envelope, opening the envelope, and then leafing through the increasingly incriminating photographs of Hawkin apparently undressing Alison was beautifully done. As a bonus, Tommy found a map of the Scardale lead mines.

This whole scene may, of course, only have taken place in George's imagination. Back in the present day, he wasn't giving Catherine the truth about what happened with the safe. Were the photographs really in the safe, or did they come from an altogether different source?

Certainly, faced with a rape charge to go along with his murder charge, Hawkin insists the photographs are fakes. And when Catherine turns up on modern-George's doorstep and tackles him about his access to child pornography when he worked vice, and his magazine article on photo montage and the manipulation of evidence, he storms out after telling her: "you have NO idea!"

Modern forensic analysis of these potentially-faked photographs is impossible after they went up in the suspiciously-timed warehouse fire but back in 1963 they are enough to convince Detective Superintendent Martin to allow Bennett to prosecute Hawkin. Tommy still has reservations though. He points out that Hawkin has lived his whole life in Scardale and apart from this one case, he's as clean as a whistle.

The chronologically estranged threads of narrative begin simultaneously to climax as George Bennett is taken into hospital in the present day and Catherine is forced to seek out the reluctant Tommy to learn the truth about the case, while in 1963, Philip Hawkin steps into the dock.

As I wrote at the start, this second part kept the pot boiling very nicely without seeming to pause for breath or leaving any single minute unstressed. The director appears to have used the cigarette smoke I commented on last week as a metaphor for the whole case, as there was much less of it in evidence this week. It's as if the smoke clears as the details of the case became less obscure. So is Philip Hawkin innocent or guilty? And if he's innocent, why has DI Bennett targetted him with his dodgy photos? It is simply because he's the most obvious candidate for the murder, or is there another reason? Was Alison actually murdered at all, or will she turn up when least expected? And what is the barely-hinted at connection between Catherine and Scardale? She visited there once as a child, she said in passing to her own daughter. If that wasn't somehow significant, she wouldn't have said it.

Final part next week at 9pm, for anyone who is still wondering, and you can read my review of the first episode by following that link.