Lots of muslins- no need for fancy ones, just a multipack for a couple of quid. It's amazing how much they dribble/ puke!
6 baby grows and six under vests at least. They are cheap and you don't want to be doing the washing all the time. If you are breast feeding, she'll have runny poo and do it frequently too; chances of seepage high.
You'll need a warm cardi to put on top of grow as it might be chilli in Sept.
Get a good baby blanket made of natural material- cotton or wool.
Couple of cotton crib or Moses basket sheets depending what you're using. I found brushed cotton ones better as you don't get that cold bed feeling when you first put them in (thus startling a sleepy baby).
Cotton wool to wash her with.
BIG tub of Sudocrem! That stuff is the bees knees.
Wipes-lots of!
Get pampers baby dry. Don't be tempted by the cheaper makes of pampers- they're not as good. A good cheap alternative to pampers baby dry in Asda's Little Angels.
What ever you do, do not buy the nappy bins that require 'special' bin bags or you'll end up spending 15 squid a go on buying the right bags!
Get yourself a decent box set as there will be a lot of time spent feeding.
Finally, buy yourself some nice creams and bath things to treat yourself.