Ring them up, tell them that you didn't realise it was an auto renew and get the money back. It shouldn't be too hard. I have done it myself. Similarly, you could ask why you have been charged £800 when a similar quote online for a new customer would be £400 and get the difference back for ease of hassle.
Insurance companies make their money these days from people who can't be arsed to switch and don't check each year. It is a pain, fortunately made easier by the comparison websites. Back in the day you had to spend an afternoon ringing around for quotes.
Good luck!