So the real underlying issues here are that you are not able to discuss money together and neither of you really seem to have a handle on your finances or where your money is going either.
£65k is a lot of debt for anyone. It's really concerning that you don't know the precise amount or what form that debt takes. How much of it is the car, for example? Does he actually own the car or is it a leasing arrangement?
(As an aside, I've been looking into dh's employer's salary sacrifice scheme for electric cars recently and even with the tax savings, it's really not a cheap way to finance a car. So I'm not sure I'm buying the argument that it's saving you money)
What would happen if you said, dh I'm worried about our finances and that we need to get a better handle on them - this weekend I'd like us to sit down and go through it all?
Dave Ramsey's Baby Steps might be useful for you (it's a bit American in parts but there's a UK facebook group which is great) or Rebel Finance School. Money Saving Expert is great for budgeting advice and making sure you have the best deals on everything.
re your previous issues with budgets: the key is not just setting a budget and making sure it's realistic but then tracking your spending in real time against it and adjusting both the budget and the spending as you go. Overspent on food shopping this month? Better spend a bit less on something else to balance it up. The other way people often come unstuck is not anticipating more occasional expenses - Christmas is a really obvious one. Ideally you need to be budgeting a little bit each month to these, so that when they roll around the money is there to pay for them.
I wouldn't even start to consider selling the flat until you have all of this under better control. It's actually very empowering taking charge of your money, you will both feel a lot better for it, I promise.
Edited to fix typos