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Thanks Ben. I followed your link which led to me this: https://duet3d.dozuki.com/Wiki/Choosing_the_power_supply
Following their calculation guide, I arrive at 500W 20.9A which includes 20% overhead (pic attached). That's inline with your suggested PSUs.

I live in the UK with 240V AC, so I will be installing a 2A fuse on the main inlet to ensure I do not exceed PSU capacity (2*240=480W). Thank you again for this awesome project! So much great stuff to learn...