I would suggest that your existing PIP is the person best placed to advise you.
The legislation says that you can only do one PIA in a lifetime, unless you have court approval etc.
In general terms, the banks are prepared to write off debt on residual debt on buy-to-lets, commercial property etc. However, many banks now seem to be insistent on not writing off debt on PPRs, and the best they will consider is a split mortgage in a PIA. They will only consider writing off debt on a PPR if you surrender the house and allow it to be sold.
Some debtors have been fortunate to have the debt reduced on their PPR's by using the votes of other creditors in a PIA.
A PIA restores you to solvency. However, solvency in the legislation is defined as being able to pay your debts as they fall due. A PIA does not have to restore you to "balance sheet" solvency.
It will be interesting to see if the courts will now take a different view in those PIA's which the creditors cannot veto. Only time will tell.
The alternative to using a PIA is bankruptcy, which is not attractive to many people.
Jim Stafford