The plug is on the wall above the wall units and I have to use the stepladders to get up there and a mop handle to reach the plug to switch it off. When the bulb is changed, I have to do the same thing to switch it back on!
You will note that . . . "before you start any work it is really important that you turn off the power to the unit at the mains. This will ensure that there’s no electricity flowing through to eliminate the potential for an electric shock."
If I'm changing a ceiling light I don't head to the fuse box first, table lamps I don't have them switched on only because I don't want to be blinded but I don't unplug them or switch them off at the wall.
I would never bother for a bulb change. Obviously make sure you only hold it by the glass end. You're not very likely to deliberately stick your fingers in the holder are you!
If the extractor is itself switched off there will be no power to the bulb holder. So you are safe Tills. No need for gymnastics. Check your old thread Tilly.
Not only that.....even if you changed the bulb with the extractor switched on, I am pretty sure that the bulbs are only rated for about 30 volts and low amps. The extractor has a transformer that steps down the power for bulb supply so the bulb supply is not fatal. Bet the new bulb says something like 30vac 3amps or something.
In general the maxim 'never trust a switch' applies to most electrical work but as long as the other lamp isn't illuminated then the fitting is safe for replacement.
You're in much more danger balancing on a stepladder and stabbing at the top switch with your broom.
IMO it should be, electrically, perfectly safe because otherwise the bulb socket ought not have passed safety regulations. Heat issues with the old bulb, and on inserting the new one, would be a different matter. But ultimately, why take the risk ? 20 seconds of non-extraction can't be a big deal surely.