studying your material
I've read that Heyer was a huge stickler for historical detail. She would study for years on end, and you get many glimpses of her extensive Regency knowledge through tidbits and comments within her novels. With Sutcliff, too, you never get the sense that she didn't know what she was writing about. Their worlds were thick and bustling and alive. They knew how what they were doing because they knew their material, and they could quickly and deftly go to work. But there was one massive difference between them for me, a difference which makes me call Sutcliff far and above the better of the two.
Time and again, when I would read Heyer's books and came across multiple allusions to the culture and the time, it was a jangling, abrupt sensation. I can't even tell you quite how she managed to do it so sharply; she might mention Jackson's saloon, or Beau Brummell, which were influential in their day - and yet, there was always something sudden about their appearance in the manuscript, as if they were there only to remind the reader that this is a historically accurate novel and that the author knows her history. The easy swing of the soul at one with the time was lacking.
But Sutcliff had that. Pick up nearly any one of her Roman novels, and suddenly you're there, no looking back, breathing the heart and soul of the same air her characters breathed. Her novels are chockablock with Roman surroundings - you can't escape the sturdy reality of Roman, which was the backbone of the ancient world in those days - and yet I can't recall being pulled up short by some otherwise historically accurate comment which left me feeling as if the spell had suddenly been broken, the dream had been popped, that I was suddenly back in my chair reading a physical book and not living the lives of the characters at all.
heart and soul
Maybe that's the key. Heart and soul. No matter how much you know or don't know about the time period, it's never going to walk and run and soar with life until you've stopped tinkering with the dead bones of it and breathed life into it, until you've given up the amateur fear of being "correct" and have let the spirit of the story take you. When a story has spirit, when it has heart and soul, the reader can see and feel it. That's when the structure of historical accuracy that you have given it has dimension just as a dream has dimension. Without heart and soul, historical accuracy is just a VBS backdrop to a one-dimensional play. You can't keep trying to compress your story into "historical accuracy" or it will grow stagnant and pot-bound and wilt. You've got to let it go. You've got to let it live. You've got to give it heart and soul, which are things historical accuracy can't give you: only you can give it to your novel.
...finally his thumb brushed over her lips and he let her go as a man lets go a wild thing he hopes might come back some day.