The replacement part arrived this morning so I was able to measure the diameter, which confirmed I should continue cutting. I measured it at 45.35mm with a vernier caliper.
Heving felt the absence of movement in the new component, the minor play detectable from the MOT was actually a bush with perhaps very little play in the rose joint, but virtually no resistance to twisting movement, I could move it full range with my fingers..
I did two grooves, and then used a chisel to bend the small piece. Once it started to move the tension was broken and the remains were ready to be tapped out.
Next job will be to clean the interior surface up, lubricate it for insertion, and then pull the new bush in using my home made puller. Assuming that goes well, then when I have done the other minor jobs she'll be ready for a retest