For General Navigation learn the formulas off by heart and be nifty with the whizz wheel.
I work better at remembering formulas than functions on the whizz wheel, so I went down that route. Other people are a demon with the CRP-5. You do have to be proficient with the whizz wheel for certain parts of the exam where there is no way of using an equation.
Get yourself on the Bristol Question Bank and use your school pass papers (Bristol were excellent.). I'd even suggest buying the General Navigation Manual off Bristol. Give them a shout. They also hold open days just for General Navigation and Meteorology. Well worth it if you think you need it.
In all honesty, the General Navigation exam is nowhere near as bad as what it first looks. You are actually only tested on a very small amount of the content. PM if you want anymore advice. I passed General Navigation and Meteorology at Oxford back in January with 87% and 78%, respectively. It's still fairly fresh!
P.S Be good at drawing your diagrams of Rhumb Lines etc.