Mains Square Adjustable Downlight

my cut out is 90 mm diameter. Will this lamp be stable, and will the surround cover the hole?
The cutout for this fitting is 80mm, It will not work with a 90mm cut out. I would recommend looking at the over sized downlights as 90mm is considered as over sized.

Can this be used with 240v? Or is it 12v low power and needs a transformer?
These are a mains fitting and so run off 240v and will not need a transformer.

Your web page indicates that this downlight can be used with a dimmer switch, but the attached data sheet seems to indicate that it is not (it does not carry the dimmable icon/image as shown in page 2). Could you please confirm that it is dimmable?
This fitting takes a GU10 mains lamp which is fully dimmable. If you are planning on using a LED lamp, then you would have to make sure that LED lamp is dimmable. Using 50w halogen lamps are 100% fully dimmable.

What is the maximum angle of tilt?
The maximum tilt offered from this downlight is 35º.

How do you change the bulbs in thee lamps - not apparent how you get the bulb out once fitted.
The inner bezel pulls out, you should be able to grip the inside of the bezel and pull it down.

What is the celing cut out for this light?
The cut out diameter for this downlight is 75mm.

Do these lights swivel as well as tilt?
These fittings only have a tilt. no swivel.