As I understand it - you're saving for a mortgage, not actually paying one off.
Just compare the interest rates and pay the money into whichever one has the highest rate (just remember that your savings will be taxed, so take this into consideration).
So, say for example, your student loan is being charged at 1.5%, if you can find a savings account of 2% ish and above - pay the money into that (assuming you pay tax at the basic rate).