He's not going to like this... I was wondering if there are any insurance products that protect people from instances like this where contractors don't do a proper job or they cause damage to a property that they're work on, does anyone here have information on that?Yes you can refuse to pay him.
Regardless of whether building regs are required or not, the stairs should be constructed to an acceptable standard. The issue with uneven risers is that they are an inherent trip hazard as we instinctively expect steps to be the same, and these will be bad enough during daylight and just looking at them, but will be worse when its darker, when you are not looking and walking slowly or trying to rush down during a fire or to watch corrie one night.
They are dangerous plain and simple and should not be accepted.