Earth bonding of kitchen sink?

Absolutely not for a kitchen sink.

However, you do need main bonding back to the consumer unit for metal services (including copper water pipes). This is installed to where the service enters the property, which in the case of water may be under the sink.

Christian.

No, but whoever's telling you (who?!) might be confusing this with a need to bond the sink to the copper pipes below the taps (would be thinner cable that 10mm - 4mm IIRC?). Whether that's obligatory or not is arguable - see lots of threads on this in the archives of this ng (Christian will argue 'no' on safety grounds, and no doubt he's right; but - eg - my electrician insisted upon it before he'd issue my safety report!)

David

No. There's no requirement to earth it, there is a requirement for equipotential bonding. I suggest getting yourself an "on-site guide" which explains the difference. They're more often confused than they're correctly understood.

(Simply put, metal things need to be conected together, but they don't need to be run back to the CU)

There is no requirement for equipotential bonding of any kind.

Only in a bathroom or shower room, and not all metal things even then.

Christian.

Well, if my reading of the "on-site guide" is correct equipotential bonding isn't required either.