In the process of having a new kitchen fitted, and being told the new
sink has to have 10mm bonding back to the CU. Is this correct? There's
no waste disposal or other mains power to the sink. Water pipes are all
copper.

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.