In our company, in which it's unlikely more than 4-500 hours per year will be flown, you need the written permission of the director of flght ops and (don't know where this came from) you must not be paid for instructing.
In practice it can be difficult to do both unless you're not married etc. and have no other commitments. Some chaps just let it slip and their SEPL ratings lapse, others with more interest in real flying as opposed to bus driving keep at it but more private flying than instructing.
I instruct and examine at the moment as well as fly for an airline but with a family I don't do more than 3 or 4 hours a month, sometimes none, and with that sort of currency - albeit with a reasonable amount of experience - you have to ask yourself how good a job you're really doing.