Right so this is something I've been poking at for years being from Hampshire originally. The official line is always "hoax, never traced, case closed" but that's a bit convenient isn't it. The signal was strong enough to override the ITV transmitter at Hannington which is - and this is the bit people gloss over - literally in north Hampshire. So you've got an unidentified broadcast quality signal strong enough to hijack a regional transmitter and nobody ever found the source? Come on.
The technical requirements for that kind of override in 1977 aren't trivial, you'd need directional equipment and a decent power source, not something you knock up in a garden shed. Whether that points to a local military installation having a laugh or something more interesting is the question I can't answer. Plenty of MOD sites in that county that were very active during that period.
Anyone actually looked into the Hannington transmitter location in relation to nearby restricted sites? Feels like that geographical angle gets ignored every time this comes up.