The Tomorrow People

A Rift in Time, Part 4: Rise of the Roman Empire (2x9)

:

John, Stephen, Elizabeth and Chris discover that the Roman Empire never ended. The Romans developed advanced technology, which enabled them to rule the world, aided by their ape-servants from the planet Trista. John, Stephen, Elizabeth and Chris return to ancient Britain, but their conversation has been overheard by Gaius, who follows them to the gladiator school. They destroy the remains of the steam engine, but they and Cotus are all captured by Gaius. Cotus tries to stop them from being killed, and is knocked-out by one of Gaius' guards. Cotus loses his memory, and Gaius and his guards all disappear. Time has been restored to its correct path.