It's probably easiest if I set the full scenario out to avoid drip feeding - one of the parties is my child but I won't say which to avoid prejudicing the answers:
Pinky and Perky are a similar age, earning a similar amount. Pinky has savings of £20k, enough for a 10% deposit, and they buy a flat together. They are advised to buy as tenants in common in unequal shares but didn't do this, neither did they sign a deed of trust, but they did both agree that in the worst case scenario of selling up they would acknowledge the unequal share then split the rest 50/50. That assumed rising property prices though!
Two years later they split - Perky is staying in the property as their mother has redecorated their room and lets a grandchild use it at the weekends. Pinky is back with parents. Both contribute to the mortgage/bills whilst deciding what to do. Perky cannot afford the mortgage alone, as is now on a lower salary than Pinky and is also having some health issues meaning they are working reduced hours. Property prices have fallen so if they sell, they will not get back what they put in after taking off fees. Pinky has spoken to the bank who have agreed that they can have a 3 month trial if they move back in, if Pinky can demonstrate affordability they will take Perky off the mortgage (and Perky will come off the deeds).
Pinky's view - although I potentially could lose money if I have to sell any time soon, I'll give Perky back the £1000 they put in plus any items that they personally bought or had already got (some of the furniture, kitchen items etc) if the bank will let me stay. If instead we have to sell, we will just have to divide up whatever cash is left in a 1:20 ratio.
Perky's view - I want my £1000 back, but I also made payments over the years to the place and put time and effort in. I think I am owed something for that. Also, when we were dating Pinky stayed over at my parents' house. Pinky offered rent but my mum didn't take it. So therefore I should get what Pinky would have paid to my mum in rent.
Pinky did not stay at Perky's parents every night, and brought washing back to their parents. Pinky and Perky bought their own food.
Should Pinky pay more to Perky? Is Pinky being unfair to come out of this with more money? Or is Perky being unreasonable asking for money that Pinky hasn't got?
Hope you have managed to get to the end without being bored or confused - I wanted an outsider's opinion :)