Changing a light fitting is the sort of job that a novice DIYer would expect to be very easy, but it actually often isn't. The novice DIYer might expect to see three wires - live, neutral and earth. But most of the time they will see a whole lot more wires than that.
With most light fittings, the ceiling rose doubles as a junction box, so you have one cable coming into the rose from the previous light in the circuit; then a cable going down to the switch and back; and then a third cable running off to the next light fitting.
We always ask handyman candidates to describe the wiring they would expect to see inside a ceiling rose - if they answer correctly straightaway, that gives us a pretty good idea that they understand how lighting circuits, and domestic electrical circuits in general are set up.
There is a good picture here of how a typical light fitting is wired.
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