I've been looking into this. If you use job cards etc, to make life easier and eliminate the need for locked storage etc. Do not input the customers address. Just input house number and postcode obviously you will need post code lookup to get the address.

Nowadays we all use a Computer to produce invoices, the way I read it all is required is to password protect logging on and maybe password protecting invoicing software.

As Graham says if a customer refuses to give there details, explain that his details are required for guarantees, and maybe MOT reminders etc. And explain that his details are not shared with a third party.
If they still refuse it's not a problem just put Customer Refused (not cash sale) for obvious reasons.

As you are aware we need to keep sales records for vat purposes, these records if you use job cards will not have customer details, so no need for locked storage.

Hope this helps

Keith