M&S dresses - sizing for tea dresses

[BCSurvivor]

I'm browsing M&S dresses online and looking at their gorgeous pink polka dot tea dress for a wedding.
I get most of my dresses from Seasalt and am always a size 16, but their dresses tend to be slightly stretchy fabric.
This tea dress has no stretch, is not available locally for me to try on, and the size 18 is unavailable online.
I have a menopausal tummy that is rounder than I'd like, so I'm just wondering if anyone can advise on whether non stretchy M&S dresses tend to be quite generous in sizing, particularly around the bust and tummy areas?

I don't find anything in M&S to be generous on the tummy or the bust.

They won’t be particularly generous, less so than Seasalt. I’d buy two sizes and send one back