Re: Hanging doors, repairing UPVC etc. You may not plan to do this, and your customer may not plan to call you in to do this, but many times the 'lock problem' you're called to address is nothing to do with the lock. At that point, you either walk away, or learn to diagnose the real problem and fix it. Common jobs are adjusting UPVC strikes and hinges, trimming swollen doors, shimming hinges, etc. You'll also get called to electric release locks or strikes connected to entry systems, so a basic abiltiy to troubleshoot an entry system is helpful.
IMO, you can't do the job well (or even competently) without a good working knowledge of UPVC hinges and strike/bolt work adjustmen, the ability to hang a door from scratch, and some basic electrics problem solving, plus a generous helping of simple mechanical common sense. The ability to confidently address a problem you've never seen before whilst bluffing that you know what your doing is also very handy!