On 02/09/2019 22:46, Mathew Newton wrote:
I need to batten out a block wall to accommodate some 75mm ducting runs and so, given the depth, will effectively be building a stud wall up against the existing wall.

I am wondering how best to secure the top plate given the presence of the joists hangers preventing me from screwing straight up into the joist, and thought about drilling a hole in the base of the hangers to allow me to do this?

Here's a picture of the hangers I've got:

http://www.newtonnet.co.uk/permanent/joisthanger.jpg

(Note that this is in another room where I've removed the ceiling, and where the existing plasterboard on the wall is still in place)

Is there any risk of weakening the hanger by drilling, say, a single 6mm hole in the base of the 'cradle' to accommodate a screw been driven up into the joist?

No problem with that. Huge safety margin.