Still ITV and still 1977 - this time it was Saturday November 26th on the Southern Television franchise covering south and south-east England. Andrew Gardner was delivering the afternoon news bulletin when the picture distorted and a buzzing overtook the audio feed at around 17:05/17:10. The picture then cleared up, with the news broadcast continuing and going on into a Bugs Bunny cartoon, while the audio feed delivered a message that was out of this world...
Although audio tapes of the event were definitely circulating in the immediate aftermath - LBC radio got their hands on a very clean sounding recording - nobody seems to know where they are now and so the actual wording of the message is disputed. (There is video footage up on youtube but I’m not 100% on whether it’s real or a recreation, possibly with one of those real audio recordings. Southern TV themselves had no tape of it, and didn’t know the signal had been interrupted until viewers phoned in.)
The various different transcripts suggest the voice wasn’t overly clear anyway. The alien supposedly delivering the message was named as Vrillon, Asteron, Gramaha, etc, by different listeners. Whatever his name, his warning that humanity was dooming itself with nuclear energy was sufficiently frightening to have hundreds if not thousands of calls clogging up ITV switchboards.
Southern was one of the only franchises in the UK open to a hoax of this kind, relying on a receiver at their Hannington transmitter rather than a direct landline. This meant that an intrusive signal wouldn't need to be particularly powerful to supersede that coming from the Rowridge transmitter on the Isle of Wight. Even so, in 1977 this would have taken a lot of specialist knowledge and equipment (6 flat bed trucks worth of batteries according to one engineer asked) to accomplish - making it all the more remarkable that those behind the incident have never been identified. My guess would be an industry insider with enough goodwill/clout for people to keep quiet.
Pulsar magazine (#5, 1978) suggested the culprits were a group who had prior experience: "On April 1st 1976 they broadcast banned records over the John Peel Radio 1 programme. On August 14th 1977 they took over radios 2 and 3 for three hours and transmitted a programme of music, made to appear as if it had come from an orbiting radio station called KSAT."
EUFOSG (Essex UFO Study Group) were more accepting that it might be a real ET...