Easy solution for the sold banks is that mortgages would follow the rates of the banks that bought the similar mortgages from the bank that has left as they would have been the mortgage holder if the mortgage was not sold.
If we remember back to the tracker issue - banks, solicitors, advisors, politicians and others said nothing legally could be done. Strictly speaking from a legal point of view they were correct.
It took a combination of Padraig Kissane, people here on AAM and most importantly someone at the head of the central bank that saw the unfairness and the underhand shenanigans of the banks to use his power to force the banks into acceding to the inevitable,
Unfortunately the central bank doesn't have Philip Lane at its helm any more.
And when the CB says the contracts don't require the rates to be offered - can they show where the customer was told that if they go into arrears your mortgage will be sold and you will be at rates far removed to what your mortgage contracts says?