As you have discovered, Persimmon are a dreadful housebuilder. As you say, even the site manager acknowledged the "painting was the worst he has ever seen" So quite why he didn't snag your house and get it all redecorated before you moved in is a question you should ask him! He is supposed to be managing the site and the standard of work!
Persimmon are rated just 3 stars in the HBF survey for a reason.
You should write to the CEO Jeff Fairburn via his PA
jacqueline.fennon@persimmonhomes.com and tell them that unless they sort it out to your complete satisfaction you will take legal action.
I hope you have legal expenses insurance as part of your home insurance to pay for this.
Contact Geoff Peter at wingrovelaw.co.uk he specialises in taking Persimmon and other housebuilders to court!
Finally. please don't duplicate posts in other catagories!