Thanks so much Caroline, worked a charm :)
I used the bottom one which greys out the options that aren't available. When I pasted the one in above it that hides them totally it still showed the Large size option even though it was set to out of stock, hid all other out of stock sizes (S,M,XL) fine but for some reason kept showing large!
The grey out option is fine though and I'm really happy to have it sorted.
Thanks again.
P.S I also have a 2px inset around vimeo embeds on my blog pages that I can't get rid of, if you have a quick fix for that too I would be really grateful!