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They are only part of the process. I've never come across one that can be relied on exclusively. Bear in mind that joists are usually evenly spaced (but not always). Quite a lot depends on the age of the walls. If it's been put up in the last 15 years you should have regular spacing of joists and wiring running only horizontal or vertical from an accessory. There is an area at the edges of walls which is allowed for wiring. That used to catch a lot of kitchen fitters out, especially since there's almost always a cupboard hanging required there.
Apparently a small neodymium magnet can be employed to find p/b nails or screws, not had to use that method but it seems pretty decent.
Airhead-
It's been done last year, in Germany though so not sure if they do things differently over here. I'll give the magnets a go as I should have a bunch in one of the 7 moving boxes labeled 'miscellaneous'.
If the magnets don't work I'll get the one @christianSpaceman linked and use his trick of several passes at different heights to be as sure as possible. If I can establish a pattern based on the detected joists I should be safe.
Thanks both!
EDIT
Well well well...
Tijmen
All the walls in our kitchen are plasterboard, as I want to hang some cabinets I think I'll need a studfinder. Any recommendations? There are 3 or 4 different models on Amazon priced around £25 and with virtually identical features, what should I look for? Or are the cheap ones all shit, and should I get a Bosch or some such?