In uk.d-i-y, Dave Plowman wrote:

Trouble is, don't such things have to have a calibration certificate to
pass regs? And getting a one off checked would likely cost more than
buying a commercial version. Unless self certification is allowed...

I expect they do. It's all about why we're wanting to "pass regs". If
it's to get a bit of paper to keep the H&S mafia or the buyer's solicitor
off our backs, a test cert for the whole installation from someone who
does the job for a living and has paid the right baksheesh, I mean trade
association membership fee, is what you need.

If, however, you're wanting to be regs-conformant so that the work you
perform is safe to your own satisfaction and so that you can with a clear
conscience assert that your d-i-y work is at least as competent as that
normally done by a practising professional, while still keeping your
spend on use-once-a-year test gear down to sensible proportions, then
knocking up something with which to confirm you didn't fall victim to
a short bout of narcolepsy while thinking you'd tightened up one of
the screws in your new sockets is an eminently reasonabubble use of
resources...

Stefek