Hi Mike, thanks for the warm welcome and advice, and it's done the trick. After the filter dropped into the top section a few times last night, I think I was worried about applying too much force to see if it would lock into place. And it did so without a issue. Problem solved.
May I also ask, was this case meant to come with a USB3.0 to 2.0 adapter as standard by any chance? I have quite an ancient mobo without the 3.0 header, and was hoping the S31 either came with an adaptor, or maybe even a 3.0/2.0 splitter?
Also, I am going to raid my old case fans and wonder which fan splitter would work with 3 pin fan connectors - I only have 2 case fan headers on my mobo? I have just purchased this one from Amazon UK, but I have a feeling it maybe the incorrect one for my needs: Akasa AK-CBFA03-45 Flexa FP5 PWM Splitter Cable http://www.amazon.co.uk/dp/B005EKU5CO/ref=pe_385721_37038051_TE_3p_dp_1
Any further advice would be appreciated. Thank you in advance.
EDIT: I forgot to mention - the screw accessories don't have a handy motherboard standoff adapter to screw them in firmly. I'm sure I'll manage when I begin my build, but they are quite small and a bit fiddly.