I have a really nice long cardi from New Look, it is very very thin though (almost see-thru) navy cotton, it came with a weird kilt pin to hold it closed which I got rid of. It is sort of floaty / drapy and it is really nice over t-shirts etc when it's too warm to actually wear a jumper whilst pushing a pram around. It looked like nothing on the hanger but my eagle-eyed friend made me try it on and now I live in it. Bought it a couple of weeks ago should still be in stock.
Have you thought of going to Debenhams or another big dept store that does a personal shopper type service? They are usually free and they know all the stock in the store so saves you traipsing round for hours.
Mind you I always find that if I have a specific 'want' I can NEVER find it when I go shopping!
I have got a great "boyfriend" style long cardi 100% cotton from Primark - loads of colours available and I am short too and it is just the right length (just past bum) but do buy at least 2 sizes too big as Primark is styled on anorexic 14 year olds IME