Offer accepted and seller/EA has asked for more money to ‘help’ with upgrades on their new home

We’ve offered and have had the offer accepted for the asking price on a house. We have no chain, not cash buyers but also not FTB as we sold our last house and moved in with parents to save on renting until we found a suitable house. After viewing a property, we offered the full asking price and had it immediately accepted. The sellers are moving into a new build so no chain there either. Sounds all quite straightforward until a few months in and they asked us via their agent to up our offer because they want to add upgrades to the kitchen of their new build. They need to find an extra £20k and need our help. I can’t believe people can be so cheeky. We said no way. Happy to stay rent free at parents for a bit longer than pay for your extras! Just posting to vent really and start a discussion on crazy sellers.