Have to wait for a moderator. Probably won't get many replies unless it's in the aptly named
Insurance forum.
For a start, go on their website and get a quote as if you're a new customer. It seems some insurance companies are simply trying it on big time (this was on Watchdog a while back) and sending people massively inflated renewals and just hoping they cba to check and just pay.
My renewal through the post was nearly £1600. Did a quote online as if I were a new customer and it was £800 something. Carbon copy r.e. my details etc. Phoned up to complain/buy the cheaper quote and they matched their own quote giving no reason for the renewal letter being so high.
Could save even more if you switch (or even just threaten to switch). Admiral/Elephant/Sky seem to be the cheapest at the moment for standard cars. Brentacre for heavily modified.