Sounds to me like they are expecting a higher intake of children who would have gone to private school, or may have transferred into other years from private school, so are trying it on. Even if a couple of families fall for it, with 20% extra from the taxman, it is worth a go!
Our primary never stops asking for money.
They charge for curriculum based trips (not allowed to do so - they can request the money and cancel a trip if they can't afford it but they can't exclude a child if they don't pay), transport to these trips (this is definitely not allowed). Any visiting companies to the school - even if they are charities providing their services for free - will have a request for payment.
I've been a bit annoyed and on a couple of occasions (knowing they are getting services for free) and have not paid, just to see what would happen. Nothing happened!
That said we donate generously through-out the year and give quite a large sum to the governors fund by direct debit so they do very well out of us. We definitely give in excess of 2k over the year with one thing and another (faith school so we do donate to Marys Meals, sponsor a child and other non-school, school fundraising efforts).
But there is no such thing as a 'parental contribution' only a voluntary donation!