We should offer to fund them having a proper financial audit, so see what happened to all that money which went unaccounted for.
Seriously, it's like a divorce - we paid more in than we got out for years, so we should be entitled to a share of whatever that surplus got invested in. Then deduct any obligation which were agreed prior to brexit.
I'm perfectly fine with no deal if they are being twats about it.
It seriously isn't like a divorce at all.
To the best of my knowledge at no point during the investment was there an expectation of getting the investment back in case the UK decided to leave the EU.
What happened is that the UK made commitments to pay for certain programs, programs which the UK is currently benefiting from and will likely benefit for some time, they are expected to keep paying for those programs for the time they committed to.
In the end it doesn't even matter though, the EU is by extremely far the party that holds all the cards, the UK realistically can do very little, it can comply maybe negotiating a few points here and there, to get this out of the way as fast as possible, so the future relationship can be discussed.
Alternatively the UK can fight this for the 2 years, and end up with no deal because there is no way that 4 decades of mutual investments can be untangled in a satisfactory fashion.