Can anyone tell me roughly what % of people doing science masters and PhDs get funding from UK research councils? i'm after some stats but they seem elusive...

That's because they are elusive! Very few people get them even though NERC say they fund 1100 postgrad students through PhDs mostly and some Masters. They are notoriously hard to get and are funded through the depts so you'll have to get in with whichever dept you want to go to assuming they have some studentships on offer! You could also be up against internal candidates as well so that will make things harder! Good luck!