Hi - Replied to this yesterday but for some reason it hasn't saved in the Forum, probably my finger trouble!
Thanks for the response. The lines are from the log file created by the cycling application Zwift working with an Elite turbo. The log file is clearly not a direct output of the ANT+ communications as they have been altered slightly before output. For example: many messages have two zero bytes added after the check sum byte; the Network Keys are all zero'd out to hide them; there appears to be an extra byte in the extended messages; and some that appear to be just wrong. So as the responses are uncompliant, they may only appear in the Zwift log file but not in the ANT communications. Also it's possible that some of the odd lines are corrupted messages, which the log file does its best to represent.
Thanks for the pointer to the extended message documentation. I think I missed this because, although I suspected extended messages, I read that extended messages start with 0x80 in the second paragraph of 7.1.1, and then discounted them as the cause. That'll teach me to read the whole section!