Who to hire to find my ceiling joists/beams?

I'd get a joiner to come and have a look. Alternatively you could look under your floorboards upstairs to see the direction and distance between them. (Probably save you a few quid and that's what the tradesman will do anyway)

You can have a look from loft room where the joists are and measure to find exactly where situated to the ceiling room

Carpenter

You would need a carpenter/builder for this to be done correctly. You can possibly do it yourself by either going into the loft or using a stud finder of some sorts. Drill a small test hole and if your drill bit comes out with saw dust on it then it's a good sign that you have most likely hit a joist (dependant on how your cieling is)

Hello, There is a few options, You could look from the floor boards up stairs. You could poke holes with a screw driver into the ceiling but this would obviously cause slight damage which would need to be filled and decorated. A service detector like a Bosch one for example, a stud buddy is also a good tool for this as it’s just a magnet that will locate the joists as they have screws fixing them. Or you could tap the ceiling, and you will hear a more firm noise when your on a joist. There are many many ways. I hope this helps. Have a good day!