Right I've had play. Have I trashed the part, maybe….
So my challenge was broadly the radius under the cylinder casting was not the same radius as the boiler. In addition, the faces on the cylinder were not perpendicular to each other.
I chose a register plane, and pulled that straight. I've then machined the faces of cylinder to parallel / perpendicular.
I've also used a boring head to re-cut the curved face. This sits nicely. on the boiler now, it it isn't parallel to the top surface.
I could put a fresh cut over it, but how thin is too thin in the casting over the boiler? its sitting at around 4mm, so I could correct it, then I'd be at perhaps 2.5mm? is that too thin for a viable gun metal casting? I've then got to sort out the fact that the machined bore is likely no longer in register to the new outer faces..
Should I just spring for a fresh casting?