I have a copy of this, recorded from the original broadcast. The special effects company that built the UFO had a lot of problems making it work.
First, the gas envelope was too baggy and wrinkled and didn't give the required illusion of a solid object. Then they found the electric motors were too large to fit the housings they had made. They intended to power it with a 25hp generator, but found the whole assembly was hugely overweight and had to resort to stacks of household batteries. They then had problems controlling it, although they eventually solved this by adding a cardboard spoiler.
The guys who built the UFO were not at all happy with how things were turning out, and this comes across very clearly!
The UFO looked pretty good on film and from a distance, but wasn't convincing close-up apparently. It made a lot of noise, which rather gave the game away. During the flight, they had seven operators controlling it (including a radio-controlled aircraft world champion), in contact with walkie-talkies. The UFO went off course, then they had communication problems and were forced to end the flight by dumping the helium.
Avebury was definitely a good choice of location. The place is a haven for dowsers and new-age nuts. The production company timed the event to coincide with a meeting of UFO enthusiasts outside a local pub, with a stooge calling their attention to it. The ruse was very obvious to them however, and none of this was ever shown or mentioned in the programme.
There was definitely some selective editing going on to make the whole thing seem more effective than it was, with accounts from the most convinced eyewitnesses only and none of those who saw straight through it. However, it made the news in the UK and Mexico and many people WERE fooled by it.