What Actually Happened?
The episode started (after a background piece where they saw the "UFO") with the discovery of an alien body floating in the river (cue Blockbuster credits sequence). Post-credits, they discovered it was actually a thin and very human with a terrible dress sense, who was now dead due to the four bullet holes in his chest.
They had the bullets analysed and found out that they were 5.56mm rounds of the sort fired by an M16. Heading upstream from the site where the body was, they found (after pumping the local postmistress for information) Starshine's decorated Ford Transit van parked outside a pub, and after some questioning of first landlord and then band established that the dead body was one Geoffrey Fossington who'd disappeared shortly before they were due to play a gig at the base's Officer's Club.
They then went to the base, but got polite and friendly non-cooperation. The base seemed jumpy. They guessed (correctly) that perhaps they were testing some kind of secret advanced aircraft. They found a bloodstain and came up with a (correct) hypothesis that Fossington had been caught up in some kind of cross-fire while standing on the base's private wharf, and had toppled into the adjacent broad (which led to the river) but that still left one question:
Crossfire with whom?
The missing piece of the puzzle came when they heard about a shot man who'd been taken into Norfolk General hospital. He turned out to be from the People's Liberation Front (Norfolk Brigade) who were hiding out in Wildwood Forest after an attempted sabotage raid on the Upper Haddon base ran straight into trigger-happy guards who'd opened fire.
The Blockbuster team then arrested the PLF(NB) finding the remaining seven members to consist of three sheep-like followers and four gung-ho hard-liners (one of whom turned out to be from MI5 with the remaining three being Special Branch, with all four totally unaware of the others' similarly undercover status).
The team then returned to their base to be greated by three high-ranking Special Branch commanders and one gentleman who claimed to be "from the Home Office" who distributed copies of the Official Secrets Act all-round saying, "This never happened... okay?"