When would first mortgage payment be taken?

[hopefully2021]

If we were to say complete mid / end of July and our agreed repayment date is the 1st, would our first payment come out 1st August or 1st September?

Probably 1st September, Directdebits take 10 working days to set up

I completed on the 1st august, and the 1st payment was 1st September

Depends on which bank. Many of them have a standard "off cycle" date around the 7th-10th of the following month if you complete in the last few days of the previous month then it will revert to the 1st September. If they did wait till September you'd have to budget for 2 months + the few days in July so that payment would be a lot more than normal

Some examples obtained from banks in this link

www.ftb.help/when-first-mortgage-payment-is-due-after-completion/

Some lenders ask what day of the month you want payments debited. We set ours to come out on DH payday. I think it was about 6 weeks from completion.

I'd budget for August but if it didn't come out until September, it'd be a bonus.

Double check your paperwork. Some lenders charge an interest only payment so many days after and then you have the full mortgage payment.

depends
you need to check with your bank.
we moved mid month and our first payment covered 6 weeks + the fees the month after.

I completed on June 10th, with payments scheduled for the first of the month. My first payment is July 16th, which is a full month's mortgage payment plus interest from June 10th. Next payment is August 1st and will be first of the month after that. That's with Barclays.