Suit jacket for big arms?

[PartyFops]

I need to buy a suit. I am generally a size 18, but I tried a size 18 suit jacket on in next and I couldn't get my arm down the sleeve .

Do you know where I can get a suit with more generous arm width please.

I have ordered a 20, but it's likely to be too big across the back.

I got a really nice one in Next which is unstructured, but still quite formal. It has three quarter arms (I've got big arms too). It is in black or floral (b/w) and there are also trousers to go with it. Sorry can't post link.