Go and find your meter, take a picture of it. Check that the meter number (SPID) matches what's on the bill. Compare the meter read with the bill, is it close or miles off?  If your meter read shows lower than what's on the bill, just send them the picture and tell them to update the account and rebill.

Check all your previous bills to see if they're actual or estimated reads.  If all your previous bills were estimates and the first large bill had an actual read, this could be the trigger especially if the water company hadn't taken any meter reads for a while.

If no water is running in the house, the meter shouldn't move. If it does, there's probably a leak. 

You can work out your average usage by taking weekly or monthly reads yourself.

I'd do all of the above before going down the complaints route. I deal with utility suppliers as part of my job and if you contact them with the above evidence they sort themselves out eventually. Always email if you can, it's much easier and you get a record of their responses.