Can anyone please tell me when the most accurate time is for a dating scan is???

Obviously i dont need my pregnancy dated lol as i am 36 weeks.
But i had a scan done at about 5 weeks, and it put my EDD date being 7th September.

I was under the impression that the most accurate time is between 7 and 9 weeks.
I had a repeat scan done at about 8 weeks and it gave me an EDD being 10-11th September.

I realise there isnt much difference, but my hospital wants to carry out a repeat c section and they are taking the EDD as the 7th sept, they want to do the c section at 38 weeks, if they use the date of 7th sept then he coupld be coming out closer to 37 weeks i dont want the baby out early of course so i really want to be armed with the best information so i can negotiate my birth plan.

Does anyone have any ideas? or any research into the best time?
TIA