I personally would of said someone saw your coding and did the oldest trick in the book "I can do better than that for less", Chris isn't that experienced of a developer himself so he would of simply gone along with it, As you can by them agreeing the work was good enough two months before.
Peter while you may not think it's satisfactory that isn't your choice, Everyone has there own way of doing things, all because it doesn't meet make you satisfied doesn't matter, Chris/sean agreed to this and paid, for them to be requesting a refund is pure slimy and well un satisfactory, all because you/whoever have said it's not up to standard doesn't mean they can do this, If I remember rightly they was refusing to pay Equinox a few weeks back because his code/system/whatever wasn't working right. There only doing to the same to this guy