It's a garage CU - probably 2 or 3W at most, and will be isolatable elsewhere. Barely more complex than the wiring of a FCU on a RFC.
Over the years I've worked for several small time developers, a few of them on multiple occasions. A semi derelict property, where I specify and install the electrics and plumbing, while they, personally, do everything else. Some of these I would happily let them do most of the second fix, while others I wouldn't let change a lamp in a lamp holder