This is when I make you quake with fear. I put M&S bras into the "cheap and nasty" category. OK, maybe they're not all the way there, but in a way they're worse because some of the bras look decent and they're cheap (£12), so you buy several of the same model in assorted colours and they last five minutes. Just Say No. Go and get yourself measured somewhere good (if Edinburgh, Jenners are notorious for trying to put people into a back size too small, ignoring squeaks of, "But I can't possibly be a 30E and that hurts and I CAN'T BREATHE!"; the personal shopper woman at Fraser is good, amongst others) and get their advice on which strapless bra will be good for you. They all tell you that a well-fitting strapless bra should be comfortable and shouldn't carve up your back into weird shapes, but in my experience that's only up to a point. That said, I'm a largish cup size and I seem to remember that you're a more normal size, so it should be less of a hassle for you; it was also a nightmare for me because I was wearing said bra with a dress that was cut low in the front, and finding a low-cut strapless bra in a 32DD is not easy.
Once you've discovered your definitely correct size, try on some different brands, and with any luck you'll find one (or even more than one!) that tends to fit. This may not be true across all brands, but I find that Panache bras always almost fit me, and they're lovely bras, especially since I prefer balconnette shapes (I've had some interesting arguments with elderly bra-fitters along the lines of, "Yes, that provides excellent support but I live in low-cut tops, it's useless. No, I mean low-cut. No, about half of the bra is going to show"), so I just order online these days as you can get £30-odd bras for a tenner. I do very well with Brastop and eBay, I'm getting my bras for between £10 and £20 and they're excellent quality. The woman at Frasers said you're meant to replace your bras every three months, which I flatly refuse to believe, but they're probably not meant to live longer than a year or so. There's also Figleaves, and oh look, they've got a sale on.
So if it's hypothetical, how can you know the shape already? Remind me exactly what Dress B needs to do, apart from be wearable at a ceilidh and fit you? At a pinch, you could borrow the dress I wore for my cousin's wedding last year, I think it would fit and suit you. I'm sure you've got other dresses, but just in case, here's a description. It's a smallish 12 (I'd have got the 10 but it was too tight over the bust, so it's fitted there on me and a shade loose elsewhere; it's the kind of dress where it doesn't matter), summer party dress rather than ballgown, spaghetti straps - er, that fit on me once we shortened them, but you're only 2" taller, indigo/dark purple ground with a design in teal, raspberry, red and off-white (sounds insane but works), floaty with a handkerchief hem and pretty panel-y bits that mean you don't have to hold your stomach in, and I have two scarves that go with (one in a sort of aqua chiffon which I personally beaded, one in a teal faux-pashmina), plus other accessories. The snag is that I wear it with a strapless bra (and with tape sticking the bra to the dress in case it gets any ideas, the picture below shows why), but if you have something with relatively thin black straps I'm sure you could get away with a strappy bra, and for ceilidhing it would probably be preferable.
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Once you've discovered your definitely correct size, try on some different brands, and with any luck you'll find one (or even more than one!) that tends to fit. This may not be true across all brands, but I find that Panache bras always almost fit me, and they're lovely bras, especially since I prefer balconnette shapes (I've had some interesting arguments with elderly bra-fitters along the lines of, "Yes, that provides excellent support but I live in low-cut tops, it's useless. No, I mean low-cut. No, about half of the bra is going to show"), so I just order online these days as you can get £30-odd bras for a tenner. I do very well with Brastop and eBay, I'm getting my bras for between £10 and £20 and they're excellent quality. The woman at Frasers said you're meant to replace your bras every three months, which I flatly refuse to believe, but they're probably not meant to live longer than a year or so. There's also Figleaves, and oh look, they've got a sale on.
So if it's hypothetical, how can you know the shape already? Remind me exactly what Dress B needs to do, apart from be wearable at a ceilidh and fit you? At a pinch, you could borrow the dress I wore for my cousin's wedding last year, I think it would fit and suit you. I'm sure you've got other dresses, but just in case, here's a description. It's a smallish 12 (I'd have got the 10 but it was too tight over the bust, so it's fitted there on me and a shade loose elsewhere; it's the kind of dress where it doesn't matter), summer party dress rather than ballgown, spaghetti straps - er, that fit on me once we shortened them, but you're only 2" taller, indigo/dark purple ground with a design in teal, raspberry, red and off-white (sounds insane but works), floaty with a handkerchief hem and pretty panel-y bits that mean you don't have to hold your stomach in, and I have two scarves that go with (one in a sort of aqua chiffon which I personally beaded, one in a teal faux-pashmina), plus other accessories. The snag is that I wear it with a strapless bra (and with tape sticking the bra to the dress in case it gets any ideas, the picture below shows why), but if you have something with relatively thin black straps I'm sure you could get away with a strappy bra, and for ceilidhing it would probably be preferable.