Thanks Chris that's really helpful. I'll need to buy some smaller heart burrs, the smallest I have is 2.5 mm so I've tried again with that. Sadly the previous two settings did not survive my attempts to salvage them, so I've done another.

I must admit I'm finding a smaller burr easier to use, even though I thought the opposite would be true initially, but just to help me understand is there a specific reason you favour such tiny ones?

Faith