New wiper arm arrived quickly and initially looked to be an exact match for the original.
However, it appears there's one significant fault with the replacement part. The spring that pulls it onto the glass is not as strong as the original and left the blade only touching at the ends. Side by side you can see it's a thinner spring.
With a fair bit of struggling I managed to swap the spring over and everything is now good.
This is the original arm and spray nozzle cover with the broken clamp and weak spring.
The tapered metal insert for the clamp was seized onto the spindle and had to be very carefully cut off in sections with a hack saw. I tried a 3 claw puller but it was going to damage the spray nozzle before convincing the insert to release.