[Katana]

I find that if I wear my bra for a very long time there are slight marks from the band however wearing a 40 band size does not give me the support I feel I need and slides around a lot.

If that's your only problem I'd say you have the right fit, at least from that brand, style, etc.

It varies even within the same brand, so you really can't rely on those calculators at all.

Yeah there's no way in hell I'm a H or HH.
I'm thinking of going up one cup size though as I nearly fell out today running for the bus when it turned up 5 mins early. But I'll probably have to wait until I see my mum so she can buy some bras for me. A good bra is normally at least £25-£40