It's always worth doing a bit of research on the comparison websites and then phoning your current insurer to see what they are prepared to do.
My insurer quoted me a renewal premium this year that was approx. 26% more than I paid last year. I knew I could get cover cheaper elsewhere, so phoned them and managed to get them to reduce the premium to pretty much what I was paying last year. It's just annoying that it seems necessary to do this; if they can reduce the premium when you phone to query the increase, why can't they offer you the lower premium in the first place?