yes you can challenge , what they say , i have done so and been succesful ....you will need to bring something new to the table though

expert advice, measurements etc will normally cause them to back down

i had a client a few years back his floors dropped and the NHBC refused his claim , so we laser measured the foor , recording every single high and low spot on a cad drawing for the whole ground floor ( timber beams ) and it was a bungalow ....they paid out 27k in the end , after a first offer of 14k

they operate just like any other insurance company .....they refuse and hope you will go away , and many do , and so they lose out

so

check your facts , get your ammo sorted and appeal their decision