If you take the flats problems out of the equation the question is do you see yourself long term with your Dp?
If you don’t then move on. Find someone else.
If you do then you need to approach this as a team and your Dp needs to cut his losses and sell. He is just throwing good money after bad.
It is a brave decision to do this but ultimately it’s a choice between living a not so great life hoping one day you can make good the losses or walk away and sell and get on with your life and probably end up 5 years from now in a much better financial position having had a better 5 years than if you and he remained in the flat.
Have a look at what people are getting for places like this flat at auctions, will this cover the mortgage. These type of flats do come up in auctions or even approach the estate agents that have sold in the same block.
If it does cover the mortgage then great, if it doesn’t then he needs to ask the mortgage company to make the negative equity bit an unsecured loan. I would show the amount it is costing now to if you both went into rented how much money could be saved and how the negative equity loan could be repaid.
Mortgage companies if they are sensible will realise that if you defaulted then getting paid back would be a pipe dream.
The amount this flat is costing each month in the rent, the mortgage and the service charge and ground rent as well as the amount needed to pay for the fire marshalls and ultimately the repairs to put it all right could rent you both a nice detached house in a nice area of the country and you would as a team be able to save so much money that even a couple of years from now with your income and his you could be owning a much better place together.
Even if you both stayed and paid all the charges, the rent, the mortgage the repairs. The flat itself isn’t going to change. It will still be a 1 bed flat by a main road which is too small for you both to wfh in.
You have to have a talk with him about your future and part of that is writing down a 1, 3 and 5 year plan. How much this flat is taking just to keep up with the payments and charges and ultimately what it would be worth minus the mortgage and what it has cost to get to that stage when everything is fixed. Then as you can live anywhere what you could rent and save and ultimately with your income as well as his, what you could ultimately buy. Probably a house without any shared ownership schemes with the extra rent as well as the mortgage and no fire Marshall’s to be paid for or ever increasing service charges.
You can if memory serves me right live anywhere in the U.K. Even the world. Why then choose a small flat by a main road in one of the world most expensive cities which is costing an extortionate amount each month and even more for an unknown repair bill that needs saving for. One thing if it was Monte Carlo and your view was of blue sea and palm trees. Quite another if your view is of lorries and chicken shops.
Even if you decide to move out and on with your life I would have a word with your Dp for his own sake to think about selling and starting again. I wouldn’t want anyone to go through what is happening with these flats.
The number of years lost waiting and saving and paying out for a tiny space in the hope things will improve and the amount realised at the end will somehow be worth it when not living there will mean you end up with more without the stress.