In your first post you said this:
She is putting on weight amazingly, she just feeds a lot, every couple of hours through the day and then this 5 constant hours on an evening. She usually has 2 feeds through the night too.
That sounds like normal BF behaviour to me (I think if you asked most peope on here their newborn would have had similar feeding patterns, except maybe more night feeds) and the fact your baby is putting on weight means you must have enough milk, or she wouldn't be gaining. Obviously you did have to feed her twice in an hour yesterday, but sometimes my baby would need two feeds in an hour, but not twice every hour. Plus, 6 weeks is a major growth spurt, it was the only one we noticed really. Your baby might be particularly hungry, some babies are hungrier than others, BF or FF, but you aren't starving her and she is getting enough, going by what you have said. She seems to be thriving on your milk. It is also possible that giving expressed milk has upset your supply and demand process a bit.
Of course, you might find constant feeding too tiring or too much of a tie and want to do something to ease that. FF or adding a formula feed might change things, or it might not. One of my sons took a bottle of formula an hour from 5 weeks old. How would you feel if you added formula or stopped BF and FF and things were still the same? I know people constantly say it will get easier, but it really, really will and that happens from about 6-8 weeks for most people.
In your position, what I would do is speak to a properly qualified BF counsellor or lactation consultant, from one of the major organisations, LLL, ABM, BFN or NCT. Not only will they be able to advise you on BF, but they will be able to make suggestions as to how to get your baby to take a bottle if you want her to, how to add a bottle of formula without upsetting your supply, if that's what you want to do and how to change to FF without any risk of mastitis etc if that's what you want to do. They aren't just there to convince you to BF at any cost.