blondieminx I'm up early so will have a go!
OK, if you're measuring at a 30E using the guides linked to on here then I'm not surprised that a 34C isn't really working. The cup volume is roughly equal to that of a 30E however, you say that the 30Es and Fs you tried on didn't work either.
Massive back fat and a gore that won't lay flat is a sure sign of a cup that's too small. The gaping at the top of the cup suggests that the style is wrong for you. What happens if you scoop when wearing your 34C? Do you get quadraboob/overspill?
You will get red marks from a correctly fitting bra but the marks should all be in one place if that makes sense? A band thats too big will ofgten leave multiple marks from moving around your back.
I'd suggest starting again! If you can't get to a Bravissimo, go back to Debenhams and try some 30 bands on back to front. If they fit as per the guide on the blog then stick with the 30 (bear in mind that some bras inthe Debenhams Gorgeous brand come up very tight in the band so may need to size up. I'd say to stick with a 30 in most other brands though). Once the back's sorted, try on lots of different styles in an E, E, F and maybe even an FF. Remember to scoop all that back fat/underarm spillage into the cup. Be vigorous!