I also have Tesco house insurance, and like you, I found that when it came to renewal time they asked for considerably more, in my case it was roughly double my first year's quote.
I got on the phone to them, and their excuse was that my previous premium was purchased at a time they were doing a special offer. I held my ground, telling them that unless they could come up with a better offer then I would take my custom elsewhere.
They eventually relented and offered me a premium virtually the same as the previous year, which I accepted.
I would say to anybody, NEVER just accept a renewal quote, always query it, unless of course it was the about same as the previous one. So 'tamborine', find out from Tesco why the increase, if they cant offer you a sensible answer and give you a quote that is acceptable to you, then go back to Go Compare.
The Go Compare website states they are covered by the Financial Services Authority, so that should give you peace of mind.
Good luck.