Originally Posted by Fudd:
“IIRC this episode was going to be part of the second run, and they moved it because they felt the first run would be too arc heavy, so I guess this is quite light hearted compared to the opener.”
It was swapped with the Gatiss episode with the scary dolls, which i must admit I was/am really looking forward to, but I think it was less to do with the story arc (because one imagines that the Gatiss episode itself must be stand alone in itself to enable the swap) and more because they felt that as it stood the first half of the series was a bit on the dark side and needed lightening up a bit.