Ok, say I ended up getting a CSM750, not the CX750.
It's an improved semi-modular, Gold-rated version of the CX750 -
http://www.corsair.com/en/cs-series-modular-cs750m-750-watt-80-plus-gold-certified-psuI thought it would have 4 separate PCIe cables but there are in fact only 2 cables, with 2 PCIe plugs on both....

Should I 100% get a refund and buy something else, or will the above be ok?
I'm at the stage now where I just want to get it powered and running.
No you will want to go trade it in for one with 4 pcie cables. You seem to be sticking with Corsair brand. If you are really wanting that brand I suggest the RM1000. You will have zero issues using it.
When your looking at pci-e cables they most likely will list each plug. So if you sort by ones with 4, you are going to get a lot of PSU's with 4 plugs but being powered by 2 cables.