Charles Tyrwhitt Casual Shirts help

[norbi]

Hi, I am 6'0 - 185 lbs, generally fit with a tiny dad bod belly ..,. I am looking to pull the trigger on a couple CT casual shirts. Now, they are all Slim Fit... my problem is their size guide with casual shirts. I am typically an L and looking at their casual shirts guide I would be an L as well, except that underneath that they have a "slim shirt" guide for casuals that comes in with a 48" chest on a L !!!!! What gives, do i size down to a Medium?? But then according to guide the neck comes in at 15.5 which again becomes small.

Has anyone worn their casual *****? and for Christs sake am I a Medium or a Large?????