Oh dear. Thx everyone for the replies. Managed to get things done - drilled out no prob. But got spooked I had mistaken the size of the old vent! So after I checked the forum this morning i put my boots on right away to go check the hole in the boler
roof, measuring tape in hand....
All good. Hole dug, butyl tape at the ready...hope to put it in place soon.
Not sure if the profile will be too low, so will keep some of your ideas in mind. Thx!