How much of "Mock the Week" is scripted vs. improvised?

Mock the Week is a British panel show where comedians make jokes about last week's events. It's made to feel as though it's all improvised banter (or at the very least jokes they told their friends over beer in the days preceding the recording).

One of the parts involves some of the participants doing a short stand up bit on a topic chosen, seemingly at random. Another part is the participants making jokes in the form of "unlikely thing to hear in whatever scenario", which often involves two scenarios given (first the one, and after it is exhausted a bit, the second). Both of these feel somehow as they are coming with these on the spot.

However! You can sometimes see that some people have notes on their hands or hidden on the desk. There are graphics prepared for some of the jokes. And in one outtakes part, a participant in the "unlikely things to hear" said something that made no sense, and then it was clarified (through a later outtake) that what he said was supposed to be in the second scenario.

All this raises the question, how much do the panelists know in advance? Do they know who is going to the stand up challenge, or what the topics might be? In other words, how far is this from a proper improv show (e.g. Whose Line)?