Thanks for the replies.
The reason I want to use downlighters instead of a single fitting is because I have knocked two rooms into one to create the bathroom, and a single fitting will not give an even spread of light in the new room shape.
Fair enough, but do you need
8 times as many lights?
Also, I like downlighters
What is it you like about them?
why do you suggest that downlighters are not that good?
First, I am assuming that you are talking about small ones, i.e. ones which take MR16 sized lamps. There is nothing whatsoever wrong with "downlights" per se, i.e. recessed lighting, but they need to be larger. The little 2" diameter ones came out of the retail display market, where they were originally appropriated from semi-professional film projectors designed to throw a narrow beam of light onto a screen and were used to throw narrow beams of light onto individual items. That's why a common term for them is
spotlights.
In other words they are specifically designed to do the exact opposite of what you want lighting for general room illumination to do.
I often refer to them as torches, and if you look at the business end of a Maglite you'll see a marked similarity to an MR16 lamp.
The loft space above the bathroom is fully accessible, and if necessary I will be fitting firehoods to cover the fittings.
Firehoods are not what's needed - they aren't sealed. You should have properly sealed enclosures over the lights, no leaks in them and no leaks between them and the ceiling, as the aim is to prevent warm moist air from getting into the loft where it will condense and start to rot the roof timbers. And the air in a bathroom is
very moist - I'll bet that even with it warm and with a fan going you will still get condensation, so in an unheated loft it would be even worse. The covers need to be strong enough to support the insulation which you must reinstate, and large enough to provide the clearances the light maker specifies.
All light fittings are IP65
Re the above, remember that the IP rating only applies to the access to the electrical components, it means that water cannot get at them. As a (ridiculously) extreme example to illustrate the point, I could make and sell¹ a light like this:
which requires a 1m diameter hole to be cut in the ceiling. It would be IP65, but it would still create a path through the ceiling for air to get into the loft.
¹
"offer for sale" would be more accurate - I doubt I would actually sell any...