Do these things first
Turn off the water at the internal stopcock if water is flowing rather than seeping. Stop using the bathroom entirely, including the WC, until you know the source. Move furniture out from under the stain and put a bucket down.
If the ceiling is bulging or sagging, it is holding a pocket of water and it will come down. It is safer to pierce it deliberately with a screwdriver over a bucket, from the side rather than underneath, than to let a soaked board collapse. Turn off the lighting circuit for that room at the consumer unit before going near a ceiling light or a downlight that is anywhere near the wet area, and do not switch it back on until an electrician has confirmed it is dry.
Narrowing down the source
Note when the water appears, because the pattern points at the source. Constant dripping with no bathroom use means a supply pipe under pressure, which is the most urgent case. Water only after a shower means the shower tray, the waste or the tiling. Water only after the bath is used means the bath waste, the overflow connection or the seal at the bath edge. Water only after a flush means the WC pan connector or the cistern feed.
Run a controlled test rather than guessing. With the ceiling still open or the stain marked, use one fixture at a time with a good gap between each, and watch. It is slower but it usually identifies the culprit in an hour without lifting a single tile.
The usual sources, in order
In the bathrooms we open up, the most common causes are a failed shower tray seal or waste, a bath overflow connection that has worked loose, a WC pan connector that has slipped, a compression joint under a basin, and failed shower waterproofing behind the tiles. Actual burst pipes are much rarer than people expect, though they are the most damaging when they happen.
One detail worth knowing: water travels along joists before it drops. The stain on the ceiling is often a metre or more away from the leak itself, which is why chasing the visible mark rarely finds it.
Getting eyes on it
Where the ceiling below is already damaged, cutting a neat inspection opening is usually the fastest and cheapest route, because the board will need replacing anyway. Where the ceiling is sound, access is often possible from above by lifting a floorboard or removing a bath panel.
We would rather make one deliberate small opening in the right place than lift half a bathroom floor. That is why the timing test above matters, it decides where the opening goes.
After the leak is fixed
Do not rush to redecorate. Plaster and timber need to dry properly, and a stain painted over while damp will bleed back through within weeks. Keep the room warm and ventilated and give it time. A stain blocking primer before painting is worth using once things are genuinely dry.
If you are dealing with a leak now across Gloucester, Cheltenham, Tewkesbury, Stroud, Newent or the Forest of Dean, use the enquiry form or call and describe when the water appears. That one detail speeds up the visit more than anything else you can tell us.