The DVD release of this episode differs from the transmitted version. The Doctor Who (1963) Restoration Team were unhappy with the scene in which the spanner is engulfed by a blast. Due to failings of 1970s vision mixing, the spanner is suspended in front of the blast for a few frames and then disappears. For the DVD release, the Restoration Team took the liberty of adding additional frames so that the spanner is blasted back towards the camera to help sell the effect better.
This episode was watched by 8.4 million viewers on its original transmission.