Look for another property !
Solicitor not giving undertaking and fact arrears paid to freeholder has alarm bells all over it.
Seller defaulted, you are buying from bank, seller doing work, seller responsible to freeholder for work.
If the seller cannot afford to keep the property, and thus he is likely, if not already bankrupt, then what makes you think he has any interest in doing any work let alone provide an undertaking. If he does, then you will be another in the queue to sue him.