Don't let it drop mate, send her an invoice, with a clear payment deadline, make it clear you do not accept the work is under par. Give her say 14 days to pay. Then send her a letter before action. Then if still no payment file a small claims case via money claim on line, the county court online system. She's taking the proverbial and trying to get free work. I hate people like this with a passion, the cow that tried to do me over got it stuck up her big time by the court system and a bailiff on her door step. Funny, she though she could rip off me and some other trades, didn't take the stuck up £&@) long to pay the others once the neighbours in her posh little village saw the bailiffs van on her drive and clamp on her car!!