1. "Their talk went round in muddled circles" means that they kept repeating the same ideas over and over in a confused (muddled) way. Their exact words no doubt varied somewhat, but the same ideas were repeated over and over.
2. "The immediacy of the blow that had been struck at them lost": probably all you need here is to look up immediacy in any good dictionary. Apparently the sudden and unanticipated nature of the blow is important in Trevor's opinion, and he is pointing out that his characters have forgotten (lost) the sense of the blow's importance.
3. "The web of detail that was their friendship" is what causes them to fail to see the significance of the the blow's immediacy. They have shared and continue to share so much information and emotion that the immediacy of the blow gets lost in that great tangle of stuff, and thus loses importance in their minds.
4. "Conversation rattling on" is easy. Literally it just means that the conversation continued on and on, but given all the context there is a connotation that the conversation was less earnest, less serious, than it ought to have been in the circumstances.
5. "And that not mattering" means that the great variety of all the things they have shared over a long time is not in itself important.
To sum up, some significant and unexpected attack has been launched against these characters, and they are discussing it in a crowded restaurant. But their discussions are not getting them anywhere because they seem to be unable to focus on an essential characteristic of the attack, namely its immediacy.
- For Teachers