Should I overpay my mortgage or invest instead?

Hi everyone,

I've been reading the UKPF wiki and flowchart, but I'm still unsure what makes the most sense for my situation.

I'm 34 and earn £62,000 a year. My wife earns around £38,000. We have a repayment mortgage with about £215,000 left at 4.45%, fixed for another 3 years.

We have around £18,000 in an emergency fund, no other debts, and save roughly £700 each month. My pension is around £65k and my wife's is about £40k. We also have about £25k invested in a Stocks & Shares ISA.

We're trying to decide whether it's better to overpay the mortgage or put the extra £700/month into our ISAs for the long term.

I know nobody can predict investment returns, but I'm interested in how others would think about this decision. Am I missing anything obvious?