New build garden issues - England

We purchased a new build, that was built by Equans, in Aug 2025. They've sorted a majorly of the snagging and we've been pretty happy with that, however they wont budge on the garden.

There was a dramatic dip toward the end of the garden which was filled in by them (at our request), however, they filled it in with excess clay and soil from the build process, with no turf, (they didnt make us aware of this prior to the work). So now the bottom of the garden is very muddy and boggy. They are denying there is a drainage issue because "their boots dont sink into the floor" despite our shoes regularly doing this and it being extremely muddy. They are refusing to fix this issue.

I am wondering what our rights are here. I know we have 2 years to report issues and we have documented everything but theh still wont budge. we've received quotes saying itll cost us £750 to remove all the soil and fix the drainage issue, which unfortunately we cant afford and doesnt seem fair considering they caused this.