I agree with @BOGGLE00 my husband always has to get a size bigger at Reiss
I would do especially on tailored pea coats as there tends to be little give. A more relaxed fit would be more forgiving. You should be okay with your usual sizing with that coat but if you're anything like me where I've trained for party muscles and are frustratingly large in the chest then I have to size up then get it tailored in the waist. This works on most items except anything with a flamboyant design or some blazers. Also check their returns policy. I remember in the past they had quite a strict returns policy which meant I used to buy the same item from their department in Selfridges as they were much more lenient with returns. I tend to just look for store specific bargains now that have no return option. Tighter fits that are too good to ignore (like the rather snug trousers I recently got for £10) became an inspiration to run my b*llocks off on a ridiculously inclined treadmill.
I'd like to buy myself a coat for xmas pressie to self (lol) and would normally be medium (38-40 chest). any idea if I should go a size large? sadly i don't have a reiss store anywhere near me so can't really see how it fits. said coat is this one: https://www.reiss.com/p/wool-blend-dogtooth-overcoat-mens-belvour-in-black/?category_id=1113&gaEeList=M - Coats & Jackets
As much as I love Reiss clothing the best discount to be had is usually on the massively overpriced coats. Sizing can be a bit on the smaller side so if you're near a store (I'm quite lucky in Manchester) there are some real bargains to be had with the odd customer returns. As a compromise on price I find some real bargains at mangoman. For your essentials (plain t-shirts, chinos, jeans), your money goes far.