Why you would get £20 commission for a lock swap baffles me
Surely a lock would only be recommended if the job genuinely required it, I would hope. Why would it need you "or the engineer" pushing or recommending for it, hence getting commission ??
Sounds wrong, It makes it look like there is an incentive to push for things that would not need be replaced, just for the commission
That's like saying every battery backup I swap, I will get £5 or £10 for.... If the job genuinely needed it, then the job must have it regardless.