We have one smallish room in the corner of our house that is colder than the rest. It has two [cavity filled] outside walls, two DG windows and it’s single storey but probably 300mm of insulation.
But there is also a boxed in soil pipe that goes from the roofspace above down into the ground. It’s just boxed in with ply and feels cold.
I’m presuming that cold air is coming down around the soil pipe and making the room cold and maybe heat is probably escaping through the ply boxing.
Any suggestions on improving matters?

Built over an Indian graveyard?

Not the boxed-in soil pipe, they are not usually cold and are sometimes warm if someone is using the shower. Warmed air will be rising from the sewer and venting out above the roof. Even if the actual case is cold, it shouldn’t affect the room temp that much.

More likely it’s because the room has two outside walls and two windows and not enough insulation above – is it a flat roof?

My climbing buddy has just moved into a flat in central Leeds with a new GF and they are freezing; the building is wedge-shaped and so they have two long outside walls of glass and steel. The heating is on full blast all the time, costing them a fortune and they can’t wait to move into an old Victorian house that she owns and is doing up.