I'm almost 6ft tall and I bought mine here. Their coats have really long sleeves. They're pricey but I think they have a couple on sale: Desiree and Isabelle. They come up really big so I'd go a size down. They're warm and toasty and waterproof - I love mine!
Definitely Next tall if you want military or fit and flare, there are some on their website, tried link but just seems to go to home page.
Does mean you have to get the Next directory though as Tall stuff only available online and I don't think you can buy from Next online without them robbing you £3.95 for directory and setting up Next account (though someone will probably come on and tell me I'm wrong about that ).