Julian wrote:

5Kw 225v
9.5Kw 218v
12Kw 214v

Have I got a case for complaint,

Where are you measuring the voltage? To avoid including voltage drop in
the house wiring you need to do it on the busbars of the consumer unit
(preferably using fused test leads), or at a point on a different final
circuit to the one you are loading.

Also take several sets of readings and average the results.

--
Andy