I actually think that a lot of the "plan", is actually part of the cover up. Not the cover up since 4th March, but an attempt to stop or mitigate something happening on that day. Let me explain:

The theory that is starting to form in my little head (which may change at any moment) is something like this:

• A nerve agent came from the US part of Porton Down, by someone or a group who wanted to take Sergei out.
• Hence the reason that G. Aitkenhead denied it could have come from "our" four walls, but looked to be terribly uncomfortable as he said it.
• The news of this "taking out of Sergei" was somehow leaked to someone else shortly before.
• The four hours in the cemetery was something to do with a meeting between those who had caught wind of it and the Skripals.
• This is why S & Y made themselves untraceable.
• This is why S was agitated in the restaurant.
• This is why there "happened" to be two doctors from Porton Down at SDH.
• This is why it was known to be a nerve agent early on.
• This is how they were able to recover - because those treating them knew what was used, as they had been told about it beforehand, and so knew how to treat it.
• D.S. Bailey was assigned to watch them as they traipsed around town, to make sure they weren't attacked.
• He failed and got there too late.
• That's why he was at The Maltings.
• It's also how he was able to get to Sergei's house so quickly (he maybe even had a spare set of keys).
• And of course it's why he must disappear and never be heard of again. Not because he's a villain or loves his privacy too much. But because if his role in it became known, it would show that someone else (our "allies" and not Russia) had tried to take out Sergei, and the Govt had desperately tried to cover this up by pointing the finger elsewhere.

I'm sure there's as many holes in that as the Govt narrative. It certainly doesn't explain the mystery couple with the red bag. But it's a start and it begins to make more sense in my little head than anything else I've heard so far.