If I were to mount a AP910c on the ceiling of my 1st floor landing (logically this is easier), will it provide coverage above, ie. for the 2nd floor landing and 2 bedrooms?

Or are they very much designed for coverage "underneath".

That is exactly where I mounted my AP910c and have had absolutely no problems with coverage anywhere in the house, unlike before when I used a normal desktop type AP which had serious problems providing even basic coverage. The house is an 80 year old one which I've always had problems with WiFi wise.
I've powered mine using POE so I only had to route one cable to it.
Even on 5GHz only it provides total coverage.

I've got a long Victorian place built when houses were made from bricks rather than plasterboard! Currently my 2925 is in the rear loft, towards the centre of house. I am hoping that adding a front AP, and maybe one at the back (rear kitchen) will give me better coverage throughout. The 2925 doesn't do a bad job, but is weak in my office, which isn't great.

Ideally I'd do PoE as well, as it makes installations neater.