I'm sure many of us have horror stories to tell. I am not a qualified elecrician but I did study electrical engineering at university so the IEEE regulations made sense, and in the past I have rewired several houses for friends or relatives. I don't any more since the introduction of part P in building regulations.
By far the worst I ever saw was an installation with one cartridge fuse ( blown and wrapped in aluminium foil), and an earth conductor which went outside to a garden tap, but had been broken en-route so was totally ineffective.
The crowning glory was a bedroom light switch arrangement consisting of (bit long winded to explain) a bayonet plug in place of the light bulb (remember those?) which fed a cable leading to a light socket from a standard lamp, with push on/ off switch so the occupier could operate the light from just above the bed. The trouble was the plastic insulation around the switched fitting had broken and had been 'repaired' by wrapping wire around the exposed terminals, this wire then lead to another bayonet light fitting holding the bulb. So basically the person in bed would reach up to operate the light switch, with hands about an inch away from an exposed live terminal
(1) In the dark
(2) with no fuse
(3) with no earth.
And they survived.!!! To this day I don't know how.
Back to the OP, I believe Damo is right. Bathroom, kitchen and exterior wiring should be done by a qualified electrician
Also any work involving the consumer unit. Other work can be done by a competent person but must be checked and signed off by a qualified electrician - at least that was the situation last time I checked but things may be even more stringent now.
K