Karl Hendy and his crew replaced our heating system last year. From the start it was one mistake after another. Wrong sized rad. Rads falling off the wall because they were only screwed into the plasterboard. Mixed up hot water and upstairs zone. Floorboards looked like they were lifted with a hatchet. Pipes laid side by side on top of joists meaning boards couldn't be put back down without resting directly on top of the pipes. Leaks which took several attempts to fix. Screwed through existing pipes. No thermostat on hot water tank. Bits of the old pipes and radiator pieces left under the floor boards. No bypass route for boiler. Motor valves installed backwards.
Luckily I spotted most of these things before he was paid so made sure they were fixed, but unfortunately I didn't notice a couple until afterwards. He promised to come back to fix them, but a year later I guess it's time to accept he's not coming.
On the plus side, communication was good, at least until the point he was paid.