From the mains service head you need to run 10 mm sq green/yellow sheathed conductors. On all the other supplementary bonding you would use 6 mm sq green/yellow sheathed conductors.
Yes, assuming your boiler has electrical connections within you will earth this.
Refer to DIY Doctor info at:
http://www.diydoctor.org.uk/projects/bo ... throom.htm
I would have said 4mm but today I was told there is no need to earth a sink.
Also not need to add any extra earths to a combi other than that which is part of power supply.
The sink may need an earth if metal waist is used.
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