If I overpay the maximum permitted on my mortgage...

[BoudiccaRuled]

And don't reduce the monthly payments, will I be charged for overpaying?
I can overpay 20% a year without being charged. But if I then maintain the original payment, I'm obviously paying off even more than 20%. Would the bank charge me for going over the limit? Or do the monthly payments they set count as separate to one off overpayments?
The bank is NatWest if that makes a difference. I know I should go through the mortgage TS&Cs first but am tired and just hope somebody can tell me 😘

It'll depend on the specific wording in your agreement, but it's usually that you can overpay a % on top of your contractual payments.

NatWest are one of the most generous for allowing overpayments so if be surprised if they charged you an early repayment charge for paying what they allow.

In our case, when we overpaid they asked if I wanted to reduce the payments immediately, or at the next annual review (about 6m away). We opted for the latter and didn’t get penalised. That was Halifax though, don’t know if NatWest is the same.

Its usually whats permitted as 'overpayment' so in addition to what youd pay otherwise.
Id just call them to confirm exact amounts rather than sifting through T&C

The 20% is usually the permitted overpayment on top of your normal contractual payments.

Check the details, but it should be the case that the 20% is the amount 'over' the contractual payments.

I paid lump sum overpayment of the permitted 10% and my payments reduced at the next annual review

I'm with Barclays but I can overpay 10%. As long as each overpayment is less than 3x my monthly payment, it goes into a "pot". This reduces the interest i pay but it doesn't reduce my monthly payments and instead shortens the length of the term.

20% overpayment would mean if your mortgage was £200k, you can pay an extra £40k per year on top of the regular payments without penalty.

If you pay more than the allowed overpayment, you will be charged a redemption penalty, the size of the penalty is usually reduces as the term decreases. That is the penalty is bigger in year 1 than during year 5. It will all be in the original agreement.