Andrew
So far everyone here is focusing on number crunching, of which there is a lot. However as unowho says, there is also a lot of geometry to consider which whilst it isn't hard I'm not sure how much is in the GCSE syllabus. You haven't said if you intend to go on to A level, which I would recommend if you are prepared to work at it, it'll give you more confidence.
My other suggestion is to get hold of any mathematics for aviators type book. This will show you what you need to know. I can't recommend one personally as I've never used one, however Bas Hamblin at Bristol Groundschool writes one and if it's anything nearly as good as his lectures on G-Nav it'll be good.
More importantly don't worry, if you want to get there you will. Whilst not having A level maths may exclude you from any sponsorship, once you have a licence it won't matter a jot.
SK