AdventuringAbout Congratulations on the toolbox! Lots of goodies in there, as you have found. You'll soon be moving on to replacing tap washers and catalytic converters! 
There's a very handy gadget in DIY stores, that can identify and warn where electric cables are buried in plaster. I've got one - to be on the safe side!
However, without one, it is fairly easy to work out where they are. They should always run vertically and horizontally - if laid by a professional. So, vertically above (or below) a light-switch or power-switch, and horizontally along skirtings/above ceilings.
That said, a few years ago I saw in a house, a really bad bit of filling diagonally across a wall, and was told " oh, that's where my husband put in an extra point"! shudder
Hard-Hat Award for AdventuringAbout and, of course... 