Under JAR ops any paid or unpaid instruction is counted as arial work and is counted in full towards duty time and rest periods and monthly and yearly flying totals.
The old CAA AOC holders have a different set of rules which allow you to instruct for free. With out it counting towards anything
So if the airline worked for is a JAR AOC holder you can instruct but it goes towards the FTL's if its a CAA AOC you can and it doesn't
Below a certain weight you can do as many hours as you like. BUT if you have an accident at work it can all be taken into account and will be brought up in court if required.
MJ