Inflation rises and markets fall...
Paying down the mortgage seems the best approach to me, you can beat the interest rate for savings and you're creating a cushion for the future.
> even with a 3.4% interest rate in my cash ISA, I am still losing buying power every month
Yikes inflation is wild right now
Just remember that inflation is over the last 12 months not forward looking. How much you have lost in real terms is 10.4% minus whatever rate your earnt in interest over the last 12 months. Let’s assume you get 3.4% in interest over the next 12 months. Then if inflation is less than 3.4% in 12 months time, you have done better in real terms. I don’t think it will fall that low but over the next 12 months I dont think those who hold cash will be losing as much in real terms as people have over the last few years.