It really annoys me in the second series when there are characters that turn up having never been seen before, and are apparently as important as people like Freeman or Ford used to be, and we're expected to think of these new people as if they've always been there. In this ep we get Collins and Grey, in The Psychobombs we get Captain Lauritzen, and, to a lesser degree, Anderson in Reflections in the Water, Turner in Timelash, and Beaver James in Mindbender.

The recent BBC 2 screening edited out an expletive of Grey's as he is trying to get a phone. Just thought you'd like to know

Just imagine how differently this episode would have run if we'd had Alec in Collins' place! Now THAT would have been a great episode! Of course, they'd have to find some way of saving him at the end, but still!!! It would have been much more believable, and very tense for the audience who already knew Alec well. And I have no idea why they brought in Col. Grey when Paul would have worked just fine as the protagonist. Dumb move all around, but the premise itself was a good one.

How come Foster can be around in SHADO HQ to get Grey's call at the end, when last we saw he was up on Moonbase, knocked out for 2 weeks?

You can see a lot of stock rcoket launch footage from Doppelganger in this ep, and in one shot of the control room you can even see Jeremy Wilkin, who had left the series by this point.

A big question is why Straker and Collins have to wait monthes for a NASA rocket to take them into oribit to repair SID. What about SHADO's lunar shuttles? Where did they go? I know Collins was supposed to be the villain in this story, but he has to rank as the most annoying character in the whole series. Straker is shown as being so strict in other eps. Why would he put up with Collins disrespectful attitude in this one?

Since it was urgent that SID be quickly repaired, an effort to move SID by remote firing of it's rockets should have been made (if it was in danger of a decaying orbit). Moonbase should have had some internal surveilance monitors which could have detected Collins sabotaging Grey's air controls. The risk of an alien entering the base would make such internal surveilance monitors necessary. Nina Berry is at her sexiest in this ep, with long hair and skirt (and not wearing her purple wig on Moonbase). Off hours, Lt Ellis wore her purple wig though.

It's also funny that all the cars usually lined up in front of the studio are also shown lined up on the backlot (including that red 60's Corvette).

How come Foster can be around in SHADO HQ to get Grey's call at the end, when last we saw he was up on Moonbase

It looked like he was on Moonbase. He was wearing the silver uniform.

A big question is why Straker and Collins have to wait monthes for a NASA rocket to take them into oribit to repair SID.

On top of that, their mission is called "SID-2". Way to keep the secret, guys!

I was watching this & because Virginia Lake was usually a replacement for Alec Freeman my first thought was to wonder what kind of script had a romance between Alec & Charlie. Then it hit me that for this episode Lake was probably replacing Gay Ellis.

Alec on the other hand seems to have been replaced by Colonel Grey for this ep.
Although if Alec had played the part the whole bloody blow to the head wouldn't have been as dramatic since you knew Alec would survive.

My guess is that this was to be the first of several appearances by Colonel Grey. He would have returned in the second season, if there had been one.

Collins looked so familiar to me that I checked the IMDB, and found out why. He was one of the several Number 2's from The Prisoner.

Until I saw this episode, I was under the impression that SID was about the size of a car. Imagine my surprise when I see Straker and Collins outside of SID, who is actually as big as a large house, or roughly half the size of Skydiver.

Straker was remarkably stubborn and naive about Collins in this episode, and lacked much of his intuition about the danger Collins represented.
I guess he can't be right all the time!

SID is seemingly knocked out for about 8 weeks, which would seem to be the best time for an alien invasion, because SID is so important for alien detection.
My guess is that although SID was finally identified as a target, no alien saucer transmitted back to base that it was knocked out and a mass attack could take place. Their home base was unaware of a major lapse in Earth's defenses.

The Interceptor had to fly all the way to Earth to take out the UFO that attacked SID, which means its speed is much, much higher than the Lunar Module, that takes many hours or a day (depending on the episode) to travel between the Earth and the Moon.

I checked imdb.com for Gary Raymond, who played Colonel Grey because I thought he looked and sounded familiar, but none of his credits are shows or movies I've seen.
However, I found a weird coincidence. A year after 'UFO', he had a role in 'The Doctors' appearing in 6 episodes. His characters name?
Paul Foster!