AnthonyL wrote:

I've got a couple of awkward bleed screws. All the screws are direct
into the back of the radiator so not easy to get at and I've fractured
a couple of cheapo keys.

As a last resort I'd like to get a stronger key - what should I go
for? Draper Zinc?

I think all the ones I've broken over the years have been zinc, I'd pick
either of the brass type that Wickes show, don't think I've managed to
break either of those

https://www.wickes.co.uk/search?text=brass+radiator+key