offset mortgage- pay off or keep the money

Hi
I have £160000 outstanding off set fixed rate ( 5 years) mortgage with £45000 in offset account

Iam paying £ 900 per month

I have another 15 years to pay

will it be better for me to contribute to savings account so that I can offset the interest I need to pay and will have the flexibility to use the money when I need OR pay off the loan using the excess money

Thanks and regards