I have been considering buying a testing plug i.e. a self contained device that plugs into a regular socket and when then runs a sequence of automatic tests. Some online customer reviews suggest they report false errors and the price range for these is surprisingly wide.

 

Are they worth it for basic diy sanity checking before calling in a pro? Any trustworthy brand?

 

 

They are OK as a very rough and ready indicator that an outlet may be wired correctly; I have one just for that purpose, as a way of doing a quick and dirty check; the three lights on the front are useful for very basic checks.  I'd not rely on one for anything other than this, though, TBH.

 

A multimeter is far more useful than one of these, and together with a safe plug adapter (like this: https://www.test-meter.co.uk/kewtech-r2-socket-adaptor/ ) can be used for checking outlets, as well as doing a multitude of other checks.  The main caution with any multimeter is to make sure you buy one with decent, safe to use, leads. 

 

There are lots of cheap Far East manufactured units around that have test leads that are bloody dangerous, in that they often have poor insulation that may crack over time, probes that aren't really very safe in terms of being adequately shrouded, or cable that has a reasonable cross section.  A working second hand meter from a reputable manufacturer is always likely to be a better bet than a no-name new meter.

Socket testers with a bleeper are also really useful for tracing circuits on your own when you’re at a fuse board as you can plug in and hear the bleeping then switch off circuits at the board and know when you have isolated a circuit. 

 

As @JSHarris says, Kewtech or Megger would be my choice.