'''I FOUND THE SOLUTION!!'''
If anyone can't find the right size Tri-wing as well, I ended up buying two different tri-wing screwdrivers (advertised as for the Wii and DS) off of eBay, they both looked identical with a red handle but only one actually fits the screws.
So if your desperate to get one all I can say is buy a few different ones from different sellers (they're only like £1 each) and hope you get one that fits.
Hope that helps anyone!