3. LAP AC/DC Digital Multimeter 600V - https://www.NoLinkingToThis/p/lap-ac-dc-digital-multimeter-600v/161fg that's the multimeter I got but when I tried to measure resistance across the live and armour it just read "ERR". Nothing in the manual about how or why that can happen...
The Screwfix site lacks detailed specifications, such as the voltage ranges/resolutions supported. At least it is CAT-II rated, but that is not quite enough if measuring at the CU (instead of at the appliance after 13A fuse, etc) See:
en.wikipedia.org
The usual reason for getting an error when attempting to measure resistance is it is seeing some residual voltage there. It could be the fault is intermittent so only manifesting at higher test voltages so it is picking up capacitively coupled voltages, or it could be there is a small AC voltage of the order of 0.1V or so induced from some other energised circuit or current flowing back via earthed metalwork in the garage, etc.
As general advice, always check for voltage before you measure for resistance! Multimeters are not considered safe for "proving dead" as there are so many ways you can get it wrong, possibly resulting in a fatal shock if you failed to detect high voltages, or an inadequately rated meter exploding if subject to a very high energy fault.
That is what the likes of GS38 health & safety advice call for the use of a dedicated voltage tester, along with some means of proving it is working before and after you checked for the presence of high voltages. This is the sort of thing you would be looking at for a low budget:
https://www.NoLinkingToThis/p/lap-ms8922b-ac-dc-2-pole-voltage-tester-with-rcd-400v/669hy
The proving units are significantly more expensive (in the £70++ range) but for occasional use you can check it works on a circuit you did not switch off before/after you test one you plan on working on.
Intelligent voltage ramp testing for voltage indicators and multimeters
www.cef.co.uk