Bought some new lights and the lathe and ceiling holes are around same diameter as the new pendant bracket drill holes 

The pendant itself will cover the holes but the bracket points won't.

 

Could I use two bits of wood stuck together? One to straddle the hole and an other to act as a packer to get to ceiling thickness?

 

May need a wee doodle if my description fails.

 

Or thinking now I type not sure if angles and thicknesses would work as it would need to be premade.

Yes. Don't that before, slid a piece of timber batten in though the hole where there was nothing to screw  in to. Works a treat, unless you are fitting a very heavy light fitting.

@Conor that's good to hear! They (three in total) arent the heaviest.

 

Any sort of timber thicknesses you'd recommend?

 

I have also spied this

https://www.google.com/aclk?sa=L&ai=DChcSEwiBhJCq_eH3AhVDtO0KHXGMAKwYABAKGgJkZw&ae=2&sig=AOD64_0H5Jf8zoQ8GgKSuDWQRty9R0X63Q&ctype=5&q=&ved=2ahUKEwiJk4mq_eH3AhWNXcAKHUzwCagQwg8oAHoECAEQDg&adurl=

Not designed for the job but also wondered if it would work.

 

Whatever you have laying around and can fit through the hole!

Lath and plaster, the bane of an electricians life.  DO NOT attempt to fit flush downlighters, been there tried to do that.

 

If the laths are still there and good and it is just the plaster that has crumbled away, I would just fill it with some quick set plaster / filler to bring it back level, wait for that to set thouroughly and then fit the light.

 

If it's upstairs and you have access use longer screws and a batten held in place from above while you fix the light fitting.

4 hours ago, ProDave said:

Lath and plaster, the bane of an electricians life.  DO NOT attempt to fit flush downlighters,

It can be done. There are downlighters that grip to the sides of the hole rather than over the top.

Cutting the lath requires patience and strengh and a good cap to keep filth off your hair, and a very sharp core drill. Even then it will snatch and twist your wrist.

Use plywood cut down into a strip instead of natural timber. That won't split when you drive 2 screws in in very close proximity.