Who is responsible for what when?

In my block one of the other leaseholders has gone bust, so the has been repossessed by the mortgage company.

All the while arrears for that flat have been building up.

At the point of repossession I guess the mortgage company become responsible for new fees.

For that owing before repossession I expect the freeholder to make up the shortfall, and make recoveries if they can.

I worry that the other leaseholders are taking up the slack from reserve funds.

Would we be expected to pay futile legal costs to chase the LH in debt?

Ta