What do I make of this wiring?
Twin brown &amp; earth is the right cable type for a switch loop. One brown is  live from consumer unit, the other is switched live to lights etc

Fans often have both a switched live and a permanent live, this allows the fan to continue to run 5 mins after the light etc is switched off. It uses a timer built into the fan. This is better for clearing moist air out and reducing chance of mould. The extra brown wire is probably for that extra permanent non-switched live to the fan.

Finding out which brown is which requires using a Cat II or better voltage tester.