Mia - I had a fitting in a v posh boutique and I think she wanted my kidneys ££ but I thanked her and looked online for something less extortionate.
I found the John Lewis video and measuring guide, which turned out to be the same measurements as the ones the posh lady had gotten, so I called Liverpool as that was the nearest store to me with a fitting service! Why I thought nearer would be better feck only knows 
She confirmed that their bras were the same size after I gave her my cms (under bust is 78.5cm/across bust is 103cm) so she said try X and Y bra, which I ordered. They came this morning. I thought the same as you, I've not put inches ON, I've LOST inches so I'm now trying to find somewhere I can get to (remembering my mobility issues, I can't drive far so would have to get the train) that's got a more reasonable range and a fitting service.
The Panache one is lovely, cup fits but the strap around the chest, under my boobage, is too lose as you suspected. Again, me thinks I've been had £££ wise. The shoulder straps wouldn't hold up a daisy in a chain! SOOOOOOOOOOOOOOOOO thin but the cup felt right. I like a moulded cup, seem free and this one was lovely. Just flimsy! 
The John Lewis own brand simply didn't fit against my chest, the cup bit where the wires go are supposed to lie between your boobage and against your sternum. They stand proud so that's going back.
Not only that, I ordered one for DD and her's is shite too!! Bloody John Lewis!
So darling Mia, and all other bra experts, HELP! I reckon I'm still a 32inch chest but have no idea of the cup, maybe a FF/G cup?
I just need to get someone, somewhere to get it right! Why doesn't someone have a company that does that for women?
Anyway, bed! Night lovelies xxx