Excessive gas boiler maintenance charge and incompetence?

I have a property rented out to tenants and am responsible for ensuring the heating & hot water work correctly. Before Christmas, the tenant contacted me and mentioned that although the heating was OK, there was intermittent hot water. As I live many miles from the property, I contacted a local company to fix the boiler and also replace an electrical switch for the oven. Several days later, the tenant contacted me and complained that the heating engineer had visited and left a disaserous state of affairs. The boiler was left leaking and the intermittent hot water not fixed, the engineer had also attemped to fix the oven switch but failed and left the switch with live wires poking out. Today (2 months later) I received a bill for nearly �300 and IM wondering what to do next. Given the engineer spent 3 hours trying to fix the fault and failed and left wireng in a dangerous state IM minded to call trading standards. Any thoughts on how to resolve this conflict gratefully received,