Not had great experience with Tesco / UK Insurance. When we moved house they told me my premium would go up because of the change of address (fair enough) so I said not to worry, and I would shop around.
I did, and found a cheaper quote. I then called back Tesco, and told them I didn't want to take on their offer and will cancel my agreement with them. Oh, says they, you can't do that just yet - you've just taken out the higher premium.
Um, no. I didn't.
It took about four weeks and multitudes of phonecalls to get that rectified, talking with person over person who didn't care they had charged my card for a payment I didn't authorise and did I once get an apology? Did I ****. In retrospect I should have taken it up with the bank and the ombudsman, but at the time I didn't think (I was moving house - I was somewhat busy). I avoid Tesco like the plague nowadays - I'm always of the belief that any company can make a mistake as they're only as good as their least experienced and underpaid staffer, but how they handle such mistakes is what matters... and they handled it appallingly.