to answer your questions:
1. What do you mean by "presenting a couple of CDs" - do you mean from the Mudcat set? Or other, personal ones? If you meant "Mudcat", then presenting a full CD would take about 75 to 90 minutes, so realistically - no chance. But I am still confused about wanting something "formal" - can you explain?
2. How many - good question! I don't know, which is why I asked that question at the start of the thread. Hopefully the responses here will give me a reasonable idea by the end of the week, and then I can plan something for the sets. It would be nice, for example to group the performances by CD. As for asking people to sing other songs than their own (in the CDs context), that is fraught with risk - which version? is the original performer happy to be substituted, who can tell me who is prepared to sing which song in advance etc etc.
3. London is big. You'll have a better chance of meeting up with Londoner Mudcatters (there are several) if you can give an indication of where you will be based.