A return to form after the poor episodes of series 5 until now, this harks back to the basics that the show got so right in its vintage years.
I too loved the references back to the show's past, and it was a neat idea to have Jonathan stalked by someone he helped put away previously (the culprit in the 'House of Monkeys' episode - although he wasn't actually seen in that particular episode). There were echoes of 'Satan's Chimney' in this episode, although this episode is inferior, I feel.
I too felt that at last Polly and Jonathan seemed to be a much better fit - this was certainly Sarah Alexander's best performance in the role. The scene where she gets into bed with a corpse was laugh out loud funny.
The striped unicorn subplot was almost as interesting as the main story itself, although anyone with a knowledge of chemistry will spot the culprit easily, as I did.
On the downside Warwick Davies tries to be funny but ends up just being annoying.