I think, from going through this with DS last year, it very much depends on the course and university about the PS. St. Andrews were brutally honest that they didn't give a fig about it and it was all grades, top top top grades, right down to the GCSE and maths challenges for Maths.
Judging by the speed of responses to his application, for Maths, I doubt whether the PS was a deciding factor.
Lancaster, where he is currently studying, and was his first choice, did however write in their offer letter how super they thought his PS was so when he did drop a grade in one of his subjects (not maths or FM obviously...) and they still took him, it can't have harmed to have written a good one.