business insurance - what do you have to have?

[gibbos3]

Please excuse me if this has been posted before.

Just what insurance does a business have to have, BY LAW?

I've heard that a very large business can insure itself for their buildings.
But, what would happen if several members of the public were injured in or around their building?
I thought every business, by law, had to have public liability insurance, but again can they insure themselves for this too? What about staff, if they are injured, they could take a business for thousands if not millions.

[gibbos3]

Thank you, Gordon Brown, Eric Mc, Jemco and JustinP1,
You have been most helpful. It's great to know that I can ask the experts for the answers. I owe you one.
Kate.