When a job like this is carried out, it’s only a matter of time before it goes wrong. This one lasted two years.
In this case, the whole house was tripping out and after a lot of investigating, it was isolated to the lights on the gateposts either side of the drive.
The cost of placing an RCD in circuit and rewiring with armoured cable at a suitable depth with ducting installed, would actually have cost less than all the hassle and costs of investigating and retrospective remedial work now required.
A small saving at the beginning, but a much larger cost in the end.
And of course, no electrical certificate or Part-P notifications from the builder either