SDLT - Is it due??

[Sticks]

The facts are:
Parents had two sons going to university. They decided that it would be a good idea to purchase a property for them to live in as both would be there for seven years.
Son's could not obtain a mortgage or afford to purchase.
Parents obtained a mortgage on their own property and used these funds together with savings to purchase the property.
The parents gifted the property to the son's on purchase.
At the same time a mortgage deed was put in place in relation to the loan the parents had taken on their property. The recital says: The Mortgagors (son's) are dedirous of buying the property hereby charged. They have not been able to secure a mortgage in their own names and have therefore requested the Mortgagees (parents) to make available to them the loan which includes the cost of purchase. The mortgagees have agreed to make the loan and have raised funds against their own property for this purpose upon condition that the mortgagors make all payments in respect of the interest charged on the loan by the mortgagees own mortgage lender.
The loan was £276,500
In effect the motgage taken by the parents was "washed through" to the sons to purchase the property.
The son's have now finished their degrees and have moved out of the property.
The property is to be transferred back to the parents by gift with no consideration being paid.
No interest was ever paid on the mortgage by the sons, technically in breach of the mortgage, in relation to the interest on the mortgage taken by the parents on their own home.
The mortgage on the parents home remains in place with the parents continuing to pay the mortgage interest on that loan.

Is SDLT payable on the forgiving of the mortgage on the property now gifted back to the parents.