Richardson and John Osborne had their big falling out over this movie. Laurence Harvey sued Woodfall because Osborne used The Reason Why as a source despite Harvey owning the rights. Osborne apparently refused to rework the script and the two had a row, which is how Charles Wood ended up finishing the screenplay. John Heilpern's bio of Osborne has some choice correspondence between him and Richardson; it's really nasty stuff on both ends.
"Do you know what lies at the bottom of the mainstream? Mediocrity!"

Director Tony Richardson was often at odds with the film's historical advisor, military expert John Mollo (who incidentally was later the costume designer for "Star Wars") over details of uniform and equipment. Fortunately, Mollo usually won. At one point Richardson wanted to dress the British Guards regiments in blue coats rather than the correct red, because he thought it would be more visually striking. Mollo threatened to quit the film if That happened. The red coats stayed.
During one disagreement Richardson snapped at Mollo, "If you want to make your own boring and accurate film you can do it when I have finished mine."

According to one version, Tony Richardson wanted Rex Harrison to play Lord Cardigan, but a newspaper wrongly reported that George C. Scott was being cast in the role. This news infuriated Harrison and he dropped out of the project, leaving the role for Trevor Howard, which was indeed a fortunate thing to happen!

You're right. Given all the problems they had with the script and shooting in Turkey, the last thing Richardson and Co. needed was an egocase like Sexy Rexy in the lead role. Howard, by all accounts, really committed to playing Cardigan; he heavily researched the part and even gave up drinking for the duration of filming, pretty much unheard of for him. I imagine Harrison would have made it more of a star vehicle.
"Do you know what lies at the bottom of the mainstream? Mediocrity!"

Rex Harrison as Cardigan??? I can't help feeling that that would not have gone well. Trevor Howard was Cardigan, nobody could have played that part as well as he did.

Richardson claims in his autobiography that he wanted Rex Harrison as Cardigan.
"Do you know what lies at the bottom of the mainstream? Mediocrity!"

Yes, Christopher Plummer's memoirs. Yes, it's in there somewhere amongst all the tedious anecdotes about alcohol and the reasons why he has such a red nose.

I just learned from an associate that the original rough cut was over four hours. I'd be quite interested what scenes may be included in it. It appears the Charge of the Heavy Brigade
may
have been filmed (going by the soundtrack's liner notes), and Laurence Harvey a decent-sized supporting role.
Christopher Plummer cast as Cardigan? I could see that but Trevor Howard's so perfect I wouldn't have anyone else play him. Do you have any source(s) for these comments?
"I had a big lunch that DIDN'T tempt fate!"