Oxford's one day course might be a good idea if you're worried but it should come together on the crammer here at bristol, also consider GTS in Bournemouth, several ex-PPSC instructors just starting on their own and eager to please. 01202 580809.
As far as INS goes Speedbird is on the right track but not completely right, what you need is an overview of the subject, not the maths. Questions asked include what is the Schuler period, what is the product of the second integration of east/west acceleration, when can you insert INS position, what happens if you turn it on and off in flight, the MCDU selections, what is a strapdown INS, what is a RLG made up of, and lots of General Nav great circle/rhumb line questions using INS terminology (DSTRK, etc.). Probably more but you get the drift
