Thank everyone so much for their feedback and time.
I have to pay £30,000 for this, I'm withholding £5,000 for the last payment. (30k including the bathroom, toilet, kitchen)
We did a ring beam not to damage the roots of the big trees in the back garden, the build is 3m away from the big trees
I told the builders I don't want a "cosmetic" job, we trusted them because we saw his builders working very professionally on another project, but I guess it was probably his own home and we thought we were going to get the same quality of work.
I saw these blocks seem to have no mortar between them?
Am I within my rights to take some money away from the final £5,000 from this work?