how do you source your tradesmen?

I have a couple of electrical jobs that are hopefully small-ish but are also slightly beyond our reach as they involve playing with the mains wiring and stuff/ potential death, and so I need to bite the bullet and get a qualified (wo)man in.

do you use yell? Gumtree? Is there an association of contractors I can look up? Basically I'm looking for someone who'll do it cash in hand, virtually buckshee/ for beer tokens, but who is qualified enough not to blow up my house when they fix the blown socket. Last guy I got off gumtree, in relation to a different issue, wasted my time before eventually realising he couldn't fix the problem- a problem I have since managed to live with, but which I might ask the new person to take a look at too (whirlpool jets, first world problems eh).

Help/ suggestions?