Hi --- Personally I would reduce the mortgage monthly by as much as is permitted without penalty in the above situation, which makes a big difference to your total interest paid over the mortgage term, forget making payments into the ISA. ! the interest you are paying on the mortgage is currently negating the savings interest....
Then consider transferring the ISA balance to a Hi Save account below for a 7.2% return, with top ups as and when you can.
http://www.icicibank.co.uk/hisave_fixedrate.html
It's better to take the tax hit on savings if the interest rate is high enough, in comparison to a poor performance tax free ISA.
Additionally if you're married/civil partnership and your partner is a non-tax payer, place the savings into their name.... utilising their tax free personal allowance, then register to receive the interest on the savings paid gross.
Thousands of investors could save paying any tax on their savings, if they were to make this simple legitimate adjustment to their investments where possible....especially when your partner can be trusted. : -- )