After years of bra fitting/shopping at M&S (blindly following my DM's recommendation that they are "the best"), I popped into a different shop to browse the bags and noticed they had a sale on underwear...went to try on some bras in "my size" and the changing room assistant offered to measure me- I could see her looking from the bra to my bust and back again. I said I was ok but after trying the bras on and not feeling comfortable with the fit, rang the bell and asked to be measured.
Turns out, whereas M&S have been telling me I'm a 42/44 D/DD for years, I'm actually a 38G! I can't believe I've been putting up with ill-fitting bras for years without even questioning it. My boobs are not resting on my belly supported, the bras don't leave grooves in my skin and my clothes seem to fit much better as a result.
The lady that served me said that the way bras are fitted has changed a lot in recent years but some shops are still measuring the old way- I didn't even know there were different ways of measuring- and that the majority of women are actually wearing the wrong size bra.
Just wanted to share what I've been told in case it helps anyone else- even if you've been measured recently, it's definitely worth a "second opinion" in a different shop!