I'm sorry that it's not what you want to hear, but quite frankly you would be mad to progress with this.
You say warranty is with Checkmate. I couldn't find any mention of warranty on Urban Splash's dreadful website - now I know why!
Checkmate are not part of the official Consumer Code for Home Builders. The Code Checkmate is quoting is NOT the same, it is their own version. Obviously Urban Splash and Checkmate do not want to be part of the CCHB such as it is,
and that is far from perfect in my opinion.Despite this, you say Urban Splash are breaching Checkmates's Code all ends up anyway!
I am sorry, but I am not going to waste time going through clauses in Checkmate's warranty or their mocked-up version of the Consumer Code. If you want to ignore my very good advice and still go ahead and buy from Urban Splash, that's your decision.
I believe this property will be nothing but trouble for you.
Finally, Urban Splash could have given you a "long stop date" of a year, yet still this would have been missed!