A disturbing episode – it would seem so easy for somebody to do this. Indeed the story seems partly based on the alleged plot by the IRA's "Balcombe Street" active service unit to poison a London reservoir. However I still cannot understand why LWT insisted this be the opening story, when 'Old Dog with New Tricks' was clearly intended to be the pilot and is a better episode, anyway.

Nevertheless this segment has some fine moments and dialogue: the "Music Hall Act" scene, of course, the interrogation of Sutton is superbly acted by all three participants and the way Bodie handles the drug-racked Susan shows that even in these early stories he was a more complex character than people usually give him credit for.

The episode moves along at a splendid lick and the action scenes are well-choreographed but none of the support characters really captured my imagination. Perhaps if the episode had been written in a way that we could sympathise with Nesbitt and his ideals it would have provided an extra "dimension".

Favourite line: "What are you two? Some kind of Music Hall act?" / "Whatever we are, you made us!"

The plot is scary and possible. Nice use of an idealistic nutcase who actually wants to do harm. One false note: the "tasting" of Susan's heroin stash – c'mon, guys! Good technique with the two interrogations: Bodie of Susan and Cowley and Doyle of Sutton. One done with kindness and some mild emotional brutality and the other with near extreme brutality! Nice cuts back and forth here.

Bodie's character doesn't have a lot of development in this one, but Doyle's nasty side comes through loud and clear: first the scene where he fakes beating up Benny but definitely intimidates the real villians in the bar and second when he deals with Sutton. The way he says "CI5" and gives poor Sutton that Look... my, my! When Cowley is actually preparing (and I think he would have) to shoot up Sutton we can see Doyle's concern. He seems willing enough to go along, though. And at the end of the show when Nesbitt, defeated utterly and wounded, lies in the bottom of their boat for the ride back, Doyle shakes his fist at the man. Ray has a multi-faceted personality!

Twice B and D go counter to Cowley's direct orders – this in contrast to their unquestioned obedience in 'Old Dog with New Tricks'. As they move into action during the climax of the story we see real physical teamwork and get to see Doyle almost fly as he crosses the wooden bridge. MS was in marvellous shape!

Bodie manages to kid around during what was a potential disaster of tremendous proportions, and Doyle chides him.

But at the end they again walk off together!

Cowley's office was constructed at the small studio complex at the Harefield Grove, just off Rickmansworth Road in Harefield. The set was retained for all thirteen first-season episodes.

The reservoir is at Thorpe Park, near Staines and just visble from junction 12 of the M25.

The building that fronted as World Chemical Products is along Slough's High Street.

The scene of the lads shooting at beer cans was shot at either Stoke Poges Hall or, more likely, the Harefield Grove studios..

The roundabout Doyle spins the TR7 is at the top of St Laurence Way, Slough.

The club where Doyle meets Benny was shot at Harefield Grove.

The interrogation of Sutton was filmed on a studio set at Harefield Grove.

As seen here, the Black Bull pub where Nesbitt injects the ADX into the beer pumps is just off St Laurence Way, Slough though its real name is, in fact, The Red Cow(!!) and has changed very little since 1977. CONFIRMED

The subsequent scene of the Austin Allegro crashing was shot in nearby Castle Street. From the roundabout walk north along St Laurence Way, turn left into Albert Street and left again into Castle Street. CONFIRMED (Thanks to Paul Fletcher-Ridley)

When Bodie and Doyle arrive at World Chemical Products, they meet some of the senior staff in Mr Harvey's office. Revealing that chief chemical analyst Mr Cummings would be able to test the vending machines for ADX, Harvey appears to pick up the telephone to talk to Cummings.... despite the fact he is stood in the very same office!?! (Thanks to "Nicolas")

Di Trevis (Susan) appeared in a great Sweeney episode, 'Bait' but appears to have dropped out of acting in the early 1980s.

Trevor Adams (Benny) was a regular in the 1970s sitcom The Fall and Rise of Reginald Perrin but quit acting in the 1990s and sadly passd away in 2000.

Penny Irving (Pam the, ahem, "exotic dancer") was a nurse to young Mr Grace in Are You Being Served? and starred in early seasons of the holiday camp sitcom Hi-De-Hi. Nothing since as far as I'm aware.

Christopher Ellison - in a small role here but later to appear in 'The Rack' - became a major player in ITV's long-running police drama The Bill as Detective Inspector Burnside. Also starred in the terrific 1986 children's drama Running Scared concerning small-time villains running inner-city protection rackets. More recently seen as a dodgy land developer in Martin Shaw's Judge John Deed series.