What exactly do you cover in an HNC? I'm not really familiar with them. Certainly on my degree it is assumed you can do maths to at least A level standard confidently and competently, although the first semester's maths was basically the bits of maths and further maths A level needed on the course being revised (self taught). It is hard, and you have to keep on top of it because it is introduced into other modules - frustratingly (for me) you can fail almost all the modules if you can't do the maths, but you don't need to be a good engineer to pass! Having said that modules like materials and design are far less mathematical, if any at all is used. As for textbooks I think the Stroud "Engineering Mathematics" and "Advance Engineering Mathematics" are recommended pretty much everywhere (though it wasn't the primary book for my course, it was said to be better for weaker mathmeticians, so that's what I went for). It's certainly good enough to teach yourself from. Important areas are calculus and differential equations (both ordinary and partial), and matrices to an extent. We've done some vector stuff but not huge amounts. If you can get your head round all of that by the time you need to apply it then things should be much simpler! (Just to clarify, that's not stuff you need to know before you start - although making sure your calculus is pretty reasonable will make life a lot easier).
Hope that helps
Ben