It’s even better than that, @seniorschoolmum!
OP, the amount added is actually 25%, because then it’s 20% of the total.
So if you add £100, tax relief makes it £125.
All the big private pension providers have an arrangement to add the automatically - they sort it out directly with the government on your behalf. So as long as you’re a tax payer, you pay in £100 and next day your account shows £125. Fab!
If you pay £50 a month for the next 10 years, that’s £1500 for doing nothing.
Do some reading, there are some very accessible articles on line. But as I said, depending on how your company is set up, there may be other ways to do it that are beneficial. The MSE website is great.