As already mentioned, start by ruling out the accessible stuff by checking all of the socket outlets fed from that MCB.
Obviously you have to make
absolutely sure the power is off to the sockets being checked, simplest DIY option is a socket tester (having proved it works in a working socket, e.g. upstairs) on each and every downstairs socket before you open it up. Never assume they are all on the same supply, someone might just have done something really dumb... Or if you can (daytime, enough light, etc) just put off the main CU switch.
If you can rule out a fault at the socket backs (e.g. cable cut on back box as no grommet, etc) then it could well be a cable fault under the floor, or some hidden junction box, but identifying that location is much harder. Generally UK sockets tend to be wired in a "ring final circuit" (though they might be radial) and if you have a suitable insulation tester you can disconnect the CU end, split the ring at a known location such as furthest socket, and then see which of the halves has the fault. Then proceed to divide and conquer until you have found the faulty link.
However, messing around with the CU is really not to be suggested as a DIY activity as there is a real and serious risk of if going badly wrong. Also you need to be aware that a multimeter is not very good at fining cable faults as they only test at a couple of volts max, and if you ever do plan on using a multimeter on mains cable always
make sure it is CAT-III or CAT-IV rated! Some cheap or old meters have little protection against a high energy fault and sadly folk have died from the resulting explosion/burns before, so it is not something to ignore.
en.wikipedia.org
Typically an electrician would have a Multi Function Tester (MFT) that includes insulation resistance at 250V or 500V, though some prefer a stand alone IR tester (often known as "megger" in the same way we refer to hoovering the floor, first big brand name). Not only are they ideal for cable fault testing, they are also designed to be safe if accidentally testing a live circuit.
Which we all know should not happen as we all follow safe isolation procedures, don't we? But sometimes in life sh*t happens and you really don't want your day to get any worse due to inadequate instrument safety.
www.eastwayelectrical.com
