Instructing according to CAP 371 is aerial work whether you are payed or not! And thus the hours go towards your total maximum flight time per month or year. It doesn't however effect your duty time.
You are meant to have the AOC holders permission to instruct on your days off. I know lots of people that don't have permission but still instruct.
The rules in CAP 371 are meant to help pilots get rest from commercial type flying. Shame they stop or hinder us using our instructor ratings when we have that sought after airline job.
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AP