Overpayments

Smifo11

Hi All.
I wondered if you could help me with a question?
I've got a fixed rate Nationwide repayment mortgage. 

I've been overpaying £266 a month since March 2019. Then I missed one, so paid double the amount the following month. So have now received a letter about the £532 payment, reducing the term. I wasn't really aware of this difference between under and over £500.

My question is : is that £3000 (ish)(made up of £266 payments) doing the same thing as one payment of £532, in reducing the term? I'm on a fixed rate, so it won't reduce the payment amount per month. Oh and is it more effective to pay £532 every 2 months, than £266 a month (as each payment is then over £500)? I rang the helpline and couldn't get what felt like a straight forward answer.

Many thanks, any info appreciated. 
Smifo11