OP, the first thing I'd do is understanding if you can increase your pension contributions.
After that, I'd recommend setting up a rainy day fund of liquid, easily accessible money that must be available for any of life's unexpected crises, from a car which breaks down and needs repairing, to unexpected redundancies, etc.
Only after that can, IMHO, the question become: how much to pay off the mortgage vs how much to invest?
Well, of course you don't need to be a Wall Street trader to understand that, if an investment yields more than the interest you are paying on a debt, you are better off investing and not repaying the debt.
Banal example: you have a loan of £1000 at 1%. You can invest £100 at 2%. If you invest, you gain £2 but spend £10 of interest, ie you spend £8. If you repay the loan, you will pay £900 x 1% =£9 of interest.
Of course it's all about risk. You are not going to find a safe investment that guarantees a return > the interest on your loan.
You need to consider riskier investments, like investing in the stock market. Over the medium to long term, it is quite possible that this kind of investments will return more than the cost of the mortgage. Quite possible, but not an ironclad guarantee.
In summary, the shorter the time horizon, the greater the risk that the investment may return less than the cost of the mortgage.
if you are 5 to 6 years away from retirement, I'd say probably repay the mortgage.
If you are >20 years away from retirement, investing in the stock market may be a good idea.