Thanks DerekO.
Nothing sinister. All mortgages are fixed rate (4.5%), with almost 2 years to run, and therefore cannot be 'part-paid'. However, they can be cleared in full but with a penalty.
I intend to use the proceeds of the sale plus about 30k of my own savings which is currently earning just above 1% net to pay off the mortgage on the PPR. Even with a penalty it would still generate savings of several k. Just checking there is no obvious reason I can't do this as if I use the proceeds to pay off the PPR I will be left with a 'mortgage' on a property that I no longer own.