Don't burn your bridges to early! Don't get me wrong I’m not saying let them walk all over you. You need to be firm but don't start throwing legal threats at them yet, at least until you know they are really trying to screw you or you will make the situation much more uncomfortable for yourself.
I took legal advice when I discovered the problem with my boat just so I knew where I stood. Have a look at your insurance there maybe a legal advice line.
You need to give them the chance to make good the problem and yes it has to be done within a reasonable time period, but something like a month isn't going to be demeaned unreasonable. You just need to push for the 2 weeks repair time they initially promised.
Info on the Sale of Goods act can be found here