Two MCB's not RCD protected, rest are with type AC RCD, and it seems the Cooker is not RCD protected and with earlier editions of BS 7671 it was not required for non socket circuits to be RCD protected and it seems originally the cooker circuit did not have any sockets.
As to testing earth leakage with a multi-meter, not really an easy task, as you need 230 volts to test with, the 9 volt battery in a test meter is no good without some thing to raise the voltage, normal is to use 500 volt which the insulation tester generates inside the meter.
We all know what we should do, but I will admit I have not done it, we should check the earth leakage at the RCD does not exceed 9 mA with no fault, I have never tested it, however I have tested that the RCD does not trip at 15 mA and does at 30 mA so that is in effect the same thing.
Four circuits on a RCD means it could be near the limit anyway, normal equipment should not allow more than 3.5 mA to earth, and the RCD should not trip under 15 mA, so 5 items left plugged in could mean your on the limit, unlikely, but possible. My PAT tester does show leakage to earth, but it is not easy to measure, the problem is inductive and capacitive linking is AC only, and the insulation tester uses DC. So EMC filters can cause a RCD to trip, but show a very high resistance with an insulation tester.
So if it trips a Plug in RCD then reasonable sure the Breville is faulty, but with the CU RCD it may have just taken the combined leakage current over the limit. I had the problem with some scales, they had an RF filter which passed too much current, we always PAT tested new equipment, as hard to return a year latter when you find a fault, and with the scales it paid off.
I am sure you can test with a multi meter, but it would likely involve danger, if you can work it out, then you have enough knowledge to stay safe, if not I am not telling you as don't want to put you in danger.
