PIR or fault find?

As the RCD is going off sometimes as well as the MCB it is unlikely to be an issue with the MCB. Seeing it and testing it would be the only way to be sure but from what you've said it might be down to an earth fault, for which there are several potential causes - a faulty appliance, a damaged or faulty accessory (socket, switch etc), a loose connection, a damaged cable, etc. You've ruled out the first of those assuming you didn't accidentally miss anything when you were unplugging/switching off all of the appliances (sometimes sockets or switches behind kitchen cabinets or appliances are easily missed). Are you confident that the rewire was done properly and do have an Electrical Installation Certificate and Building Control Notification for it? If so I'd recommend the fault finding route. Fault finding is usually done on an hourly rate and the cause of most faults can be found within a couple of hours or less (I would provide your electrician with a copy of the Electrical Installation Certificate if you have it before they start as it may help them to be able to compare their results to the original test results speeding the process up and saving you money). Depending on the issue it may cost extra to get the fault rectified. An electrical installation condition report usually covers the whole electrical installation and follows a specific format. It is a good thing to get done every 5 to 10 years (or perhaps more frequently if you're a landlord and the tenants change regularly) but it doesn't include investigation of the specific causes of faults and only a sample of accessories are looked at so it might not be possible for the electrician to identify the cause of a specific fault as part of the report.