Ive been playing flute for 5 years and i am taking grade 5 at christmas and last week started saxophone, i dont want to let my flute slip as i still want it to be my main instrument. Im hoping it wont effect my flute playing or my tone. Does anyone have any tips or ideas on what I can do?

If you want the flute to remain your main instrument then it is up to you that you make sure it is. Don't spend more time on the saxophone and neglect the flute.

I don't really know about the effect on tone as I went from Sax to Flute, but I am managing perfectly fine.