The amount you have isn’t the thing that interests me - because we can’t know without more detail on your life so far whether that’s an excellent start or a hugely missed opportunity!
What interests me is that at 35, your focus on your pension seems to have been prompted by your parents, you had to look up how much you had, you’re not confident on compound interest, and you’re not confident about your decisions for overpaying your mortgage.
For me the “ideal” pension situation at 35 is that when your parents ask (and fair play to them, they care) you can say, “I’ve got abc and my plan is xyz for reasons 1,2,3”
And I think you’re close to that! Fair play that you were able to log in, that you understand there’s a balance to be struck with mortgage over payment, and that you know compound interest is a thing.
I would say that more important than your next £10K saving, is your next bit of financial education 🙂
Get yourself over the MSE, check out the intro articles, and get excited about learning, planning and saving 👍🏻