This is a good, basic and simple RCD tester. It is powered via the device under test and will hold a reading for 30 seconds or so after the device trips. To avoid tripping upstream RCDs and making yourself unpopular by cutting power to the whole ring main, you will also need an isolation transformer (which would be connected between the main supply and the RCD to be tested).
You simply plug the RCD in, plug the tester into the RCD, verify wiring polarity using the two built in neon indicators, select the RCD current (usually 30 mA) and select the test current (half, x1 or x5 rated current). If the RCD trips, which for x1 and x5 it ought to, you will get a reading of how long it took. You run two tests (0 and 180 phase) and take the higher of the two. That's it.
Handy to have if you do PAT testing as most PAT testers don't have RCD test capability built in, unless you bought an expensive one. You can get one of these Robin testers around the £20 - £30 mark second hand depending on age and condition, so it's worth getting one for your toolbag.