JA
According to a couple of forums, the reason 21/3/85 went missing is apparently because in 1993 they sent the 1" master to a transfer facility to be digitised onto D3, but it went missing. I don't know if that's true, but it sounds plausible- I wonder how many other things have gone missing in this way. I'm presuming there's no raw studio tape or mute links version still existing they could have used to build up a broadcast quality version, both of which BBC4 have done with TOTP episodes in the past.
Doesn't explain why they couldn't show the VHS copy when they've shown such things before though.
Doesn't explain why they couldn't show the VHS copy when they've shown such things before though.