2004 Rover 25 water in the boot well. what should i be checking?

Go in boot with torch and get someone to hose rear of car , should then find leak

Had same problem but that because i've change aftermarket tailights. Make sure the soft surrounding bit between the tail light and the chassis has no gap and the screw are nice and tight. I use two soft bit overlaping each over to give maximum water resistance. also, my mate hit me at the back so the little airhole behind the numberplate inside the boot has a slight gap and water got in the gap. sort it with duct tape. Cheap repairs but gets the job done. Make sure you get a jug or hose and spay the water on back side to see if any water get in.

rear screen wash pipe leaking in hatch comon fault on rover bubble

I had a similar problem with a 416 hatchback. Couldn't find the leak anywhere until I looked one day when it was raining very hard. Dropped the back seat and stuck my head in from the passenger compartment. I then saw water dripping down from inside the roof pillar into the boot. Just a tiny amount, but there it was. On inspection of the hatch hinges, very small cracks in the paint around the edges could be seen. They would hardly be noticed. I painted around the hinges with Autoshield paint, the stuff you use on the underside of sills and floors (the paint is black, but you can't see it when the hatch is closed). This solved it for me.