Just to add that testing in effect serves two different purposes - it verifies the design (e.g. the cable isn't too long) and it verifies the installation (wires connected to the right terminals etc.) If R1+R2 is OK, then from a design perspective the cable length is OK and (excepting vary rate instances of reduced c.s.a. N) then the design for the N side must be OK too. It's nigh on impossible to create a resistance of several Ohms by an installation error - either it's connected or it isn't. A loose connection either rattles between connected and not connected or poses a very small additional resistance (usually very small fraction of an Ohm). That small fraction of an Ohm, on a circuit carrying heavy currents for long durations may well overheat - eventually oxidising metal and charring insulation which can lead to lead to higher resistances - but that really only gives you a means to detect the problem after the event, which isn't a great help for testing that's usually done before the installation/changes are put into service.

    - Andy.

Just to add that testing in effect serves two different purposes - it verifies the design (e.g. the cable isn't too long) and it verifies the installation (wires connected to the right terminals etc.) If R1+R2 is OK, then from a design perspective the cable length is OK and (excepting vary rate instances of reduced c.s.a. N) then the design for the N side must be OK too. It's nigh on impossible to create a resistance of several Ohms by an installation error - either it's connected or it isn't. A loose connection either rattles between connected and not connected or poses a very small additional resistance (usually very small fraction of an Ohm). That small fraction of an Ohm, on a circuit carrying heavy currents for long durations may well overheat - eventually oxidising metal and charring insulation which can lead to lead to higher resistances - but that really only gives you a means to detect the problem after the event, which isn't a great help for testing that's usually done before the installation/changes are put into service.

    - Andy.

Thanks for the reply Andy. 

I agree with most things you have said but I don't like assuming. Like you said "it must be ok" I just think a simple R1+RN check at the end of the circuit would confirm this and would have thought this simple check to make sure the RN side of the circuit was in good condition. This would then allow us to check the values against a set of values set by the regs so we know that it will trip in a short period of time. Also having this value could help confirm volt drop if required, but this is obviously not a thing. 

For the example you gave with the high resistance on the neutral - If you went and carried out an EICR on a property and that had happened on a neutral of a radial it would not have been picked up as there is no R1+RN test. Also, say you did test it and it came back with 10 ohms, we both know that's incorrect and must be something wrong but there is nothing to say that value is too high for that circuit is there.

The rules do not say never measure Rn, just that you do not have to.

do realise that a 10 ohm resistance is far too high for a single point fault -  even at just one amp it dissipates 100 watts and if that is something the size of a terminal screw it will soon heat up to molten metal and in doing so either blow clear, or weld itself into a low resistance state. The former will be found and the latter will now work for years without incident . The sort of extra resistance that leads to slow cooking is that which dissipates a watt or two in less than a cubic cm , and those sort of resistances do not really show on test, as they are comparable to very short lengths of cable.

As Andy notes, things are usually either connected or not, though some things that measure open circuit at low voltages connect at higher ones, as thin layer of oxide are blown away or small gaps are jumped by arcs. (Though you do not really get long arcs with copper, other materials such as carbon, or indeed welding rods (!) can be persuaded to strike arcs of several mm with 230V. To start an arc in air with 230V needs things to be closer than 20um (Pashens rules apply more or less), but once struck the burn clear or weld solid rule applies.)

This is why twisted joints work as well as they do - the initial contact area is almost nil, but soon expands once current  flow and at a microscopic level the peaks are welded to the troughs...

Mike.