Yes, Christine is on the CS "On air" CD. You're right, they are live BBC recordings. Unfortunately, the CD liner notes don't indicate which songs were recorded when so you'll have to do a lot of guessing as to which ones she's on specifically but naturally she does sing lead on IRGB, GLYUTB, "Hey baby" and a track which is billed as "Night is when it matters" but it's actually a live recording of the song "No road is the right road" which you'll remember from the CP album. I would imagine she is playing on some others too like "Tired eyes" etc. but it's pretty clear that she is not on later more rocking CS material such as "Telling your fourtune" for example and songs like "Tears in the wind" & "Things you put me through" were the first songs to feature Christine's replacement in the shack, Paul Raymond, who later with shack bass player Andy Silvester went on to play with Dave Walker in Savoy Brown, Danny Kirwan on "Second chapter", etc.
There is another live CS CD you'll see in the Penguin discography called "From the vaults" which is also live CS material presumably from different eras and once again the liner notes don't say when these recordings are from either but at least they are different recordings then all of those which appear on the "On air" CD except "You've done lost your good thing now" (which is billed as "You done lost that good thing now" on the "On air" CD but it is the same exact recording on both CD's), apart from that though all of the recordings are unique to their respective CD's. Anyways, since the notes don't say when the songs are from you'll have to guess again as to which songs she's on but she's obviously on "When the train comes back", "It's OK with me baby", "Strange things happening" & "Mean old world" from FTV as she's singing lead on these. I'd imagine that she is playing on some of the others too though such as "Night life", "Tell me", "Lonesome whistle blues", "The letter", "San Ho Zay", etc. but who knows for sure?
So, I hope this was of some help.
John