Final result!
So my cables arrived very quickly, and my splitter just arrived today.
I've connected them up, and it's working perfectly! Also the splitter sure is a "Tratec" Splitter with the specific cable modem and set top box labelled outputs.
Thanks so much for all your help!
If anyone else is having issues with distortion, using cheap splitters, or is thinking about getting the H2H for cable, here is what I'm using and what I fully recommend:
Tratec EcoLine Splitter (£5.75) - just realised its ended, but if you're thinking of getting any other low cost splitter, then DONT!! Find one of these!!
Coaxial Cable - Same as what VM used! (£2.90) - I purchased 2!