Not enough information given.
Are you in a deal that has exit penalties?
Is the mortgage interest only or repayment? It sounds like you have an IO mortgage at 6%, which is quite high. How long have you had it and what was your original plan to pay it off?
What is the interest rate? It sounds high - do you have other debt/a poor credit history? If so, you'd probably be best paying that off, building an emergency fund, then maybe over paying, but also making sure you're on a decent rate.
How much is in your ISA?
What sort of ISA is it? Why is it 'not earning anything'? Mine has grown about 15% in 18 months and it's just bog standard tracker fund.
Would you be left with an emergency fund/money for planned large purchases like car or home improvements if you did pay it off?
What would you do with the £300 if you didn't have a mortgage? Do you need it for essential bills, would you save it, or would it get spent on fun stuff?