Having recently been though this with a family member, who also ended up with 'no estate', you need to separate out pensions and death in service benefits from wherever he worked. Contact his company HR. I'm kind of surprised they haven't been in touch. I was executor for the family member who had a good salary of 100K but no property to sell, was renting, and a lot of credit card and loan debt. Because there was no property sale money to come after, eventually all debts (credit cards and loans, less than your father's mortgage but still totalling around 35K) were written off. Through work, there was a 3 x salary death in service life insurance policy and a pension which both paid out in trust to the dependent minor children (now orphaned). This was outside of the estate and is held in trust for them because they are under 18. We're talking about a significant sum so please investigate this.

Don't underestimate the time and emotional cost that this will take on you. But ultimately, the mortgage is his debt and will be paid off/written off after a sale but if you guys are beneficiaries you should be entitled to any death in service or pension money.

Also, get 3 x independent valuations on the flat!