I think it is unfair that they agreed £2,800 and are now going back on this. But could they be feeling bitter about giving them so much money? Or maybe something at the wedding was not to their taste and they are being fickle? Once you say you will do something, you should, whether offering to do so is right or not.
Maybe they decided £2,800, not £6,500 (half the cost), because of all the other stuff they have kindly paid for.
This couple are so lucky. So many couples are struggling to pay rent, never mind find a substantial deposit for a modest house, with houseprices how they are. They've had new windows paid for which isn't cheap, and if their boiler ever needs replacing then they'll have a new one paid for as well.
I'm pretty lucky financially, but no thanks to either set of parents, all our own doing. Both sets of parents gave a gift of £250 towards the wedding/the house, but we made sure we could afford to marry before we did so, and then spent only what was necessary, and the £250 was a gift, not a contribution to the cost.
I also think it's a sweeping statement to say that most parents pay for the wedding between them, á la Father of the Bride style. That does happen but not as often as is thought. Friends of ours were in LTR and fell pregnant; they could not afford to marry and buy a house, so they decided to buy the house for security for their child. They will probably marry in a registry office. None of our parents offered to pay for parts of the wedding, although my gran ame with me to a dress fitting and decided to pay for my veil.