A crossover I actually wrote a fanfic bit about (many years ago, don’t have the pages anymore): the TARDIS materializes in London, circa 1966 - in full swing mode at that time. The latest rage hot spot is a club called “the Naga Box”, and a peculiar effect is taking place. All the hormonally-crazed young mods who frequent it are turning into super-serious, emotionless prigs. In time, these now puritanical youths each explode into bezerk fury at some inappropriate time after their transformation.
This arouses the curiosity of the Doctor (#7) and Ace (I wrote this at about the time the Virgin New Adventure novels were coming out). But also investigating the mystery are a certain pair of secret agents - a man known simply as Steed, and his ultra-cool partner Mrs. Emma Peel. Initially, the Doctor & Ace, and the Avengers each suspect each other of being part of the problem. However their seperate investigations converge when they alll discover the true culprit - the Rani, using a similar scheme that she used during the Luddite revolution (removing essential chemicals from the brains of club-goers, which eventually drives them insane).
The Rani has opened a time corridor that connects the Naga Box club to her labs on the world she rules. She uses Cybernaut drones as her workers. When the Doctor, Ace, Steed & Emma discover this, a fight ensues between them & the Cybernauts. Steed & Ace are accidentally thrown through the time corridor, which is then disconnected.
The Doctor takes Mrs. Peel on a TARDIS ride to the Rani’s world, and they are forced to make their way through a perilous jungle full of man-eating plants to get to the Rani’s lair. Meanwhile, Steed & Ace successfully escape from a death-trap that the Rani has left them in. All four heroes meet up, only to be seperated once again - this time it’s the Doctor & Steed, and Emma & Ace who have to work together.
Any way, making a long story short - the good guys triumph, and the Rani’s plans are foiled. The foursome return to Earht, where the mods & birds at the Naga Box are once again shagging most improperly. The Doctor & Ace leave, while Steed & Emma decide to have a nightcap at the Box. They strike up a conversation with a pair of scenesters named Polly & Ben.
And had I’d known about “Sapphire & Steel” at the time, I’d have worked them into the plotline somewhere.