If your mortgage is at 5% interest, and you over pay £50/month, you are benefiting in 3 ways.
Firstly, that is £50 less on your overall mortgage you need to pay at the end. Over a year that is £300, over two years it is £600 which (depending on the amount of your repayment) may mean you repay your mortgage 1 or 2 (or whatever) months earlier than the current final date.
Secondly, any extra repayment you make goes to the principle you owe, and reduces that. Meaning the interest calculated each month is less. £50/month at 5% is a reduction of approx 21 pence in interest payable each month. Meaning this 21 pence also, comes of your principle loan, meaning it isn’t just the £50 above, but now an additional 21p.
These amounts might sound small, but the power of compound interest means they add up significantly over time.
Thirdly, by making these extra repayments and reducing your overall principle loan amount, you own more equity vs the loan in your home, and puts you in a better position to renegotiate loan terms and rates in future, if needed (ie getting a lower interest rate).
I say do it! From little things, big things grow!