Wetsuit Sizes

Just after a wee bit of advice on my new wettie, it's a rip curl f bomb 6/4 and i'm stuggling a bit with the size. going by the chart I'm a m/s as i'm 5'8" and 11.5 stone/73kg but when I tried one of them on I couldn't move in it ( I was sort of walking like c3po off star wars!
not good) I explained this to the bloke at the shop and he suggested a l/s which I've now got and tried on and it seems to fit a lot better except there's a bit of looseish neoprene under my armpits and absolutley no contact between the suit and the small of my back. It must be sitting off about 1/4-1/2 inch in the middle!
The thing is it seems to fit quite well on the rest of my body so I'm just not sure if it's the right size or not?
Any advice would be great, cheers, Brian.

The thing is it seems to fit quite well on the rest of my body so I'm just not sure if it's the right size or not?

Any advice would be great, cheers, Brian.