You do realise that voltage drop only occurs when there is load on the conductor, infact modern multimeters have a practically infinite resistance.
You could attach wet string to a multimeter at one end and 230v at the other and it would show 230v (probably and this is not recommended), the problem comes when you apply a load to the wet string; then the impedance of the string will gang up with the impedance of the cable you have connected it to, the load, the transformer and the generator; and of course very little current will flow. Your faithful multimeter will then show the voltage across the string falling like a stone.
Fortunately, rather than guessing a cable size, installing it and then applying load and measuring the voltage drop; we can use tables to work the voltage drop. Resistance and thus impedance doubles every time we double the length, so a 1meter long cable will have twice the impedance of a 2meter long cable. Normally of course this isn't a problem, but over long runs it can be. Fortunately impedance is also proportional to conductor area, well roughly anyway (skin effect can occur in large round cables), so by choosing a larger cable we can reduce voltage drop.
Of course the load is just as important as the length of cable, a 1amp load on a 2 meter long cable will show the same amount of voltage drop as a 2amp load on a 1meter long cable.
Pardon the rant, but this isn't the whole story, but I thought it might make you think about what exactly you need to do.
So should I measure the length of the cable run from the CU or the socket for the spur? The house is probably 10-15m long (CU is at the far end). Has there already been some voltage drop by the time the cable is run through the house?
