Unlike the NHBC warranty which has a minimum claim value of around £1,500, below that they won't do anything, the LABC and Premier Guarantee (same warranty in all but name) have an excess of £1,000 that buyers must pay the first £1,000 of the cost of any claim.
There could be many reasons for the creaking noises. If you search this forum for noisy floors etc.
What is wrong is the builder's comment "nothing can be done"
We are talking about a house here, not someone with terminal cancer!
Of course something can be done but it may be expensive, time consuming for the builder and inconvenient for you.
Your best bet is to continue to complain to the housebuilder. Under a normal new home warranty, housebuilders are responsible for fixing defects for two years, irrespective of the shared ownership 1 year DLP.