Welcome, if I was you I would put more than a disc lock on it, if your keeping it in your garden. I would buy a few heavy duty chain and padlocks to secure it to a solid object like a pole or a metal fence. Your insurance quote seems about right. If you have used a compare website then thats the best your going to get. MCE isn't that bad either. They give you free breakdown cover and insurance on your helmet and leathers. You could always go third party only, but then if your bike gets stolen, your knackered.