Get the kn*bhead sparks in who designed, fitted and 'tested' it back. It's evidently not FFP.
An interesting point is that if (as seems fairly probable) the problem is a duff MCB in the house CU (which trips whenever a significant load is applied to the circuit), I'm not at all sure that the standard regime of I&T would have picked it up - because, IIRC, the one thing (which one might think was pretty basic/'obvious' - perhaps the
only 'test' that many DIYers would undertake) that the prescribed I&T regime does not include is applying a significant load to an energised sockets circuit! If I'm right, the recognised testing regime checks all sorts of things, other than that the circuit is actually 'fit for purpose' (i.e. satisfactorily supplies power to a connected load) - which, if true, is a bit ironic!
Kind Regards, John