I take it Company names mustn't clash, but trademarks can if they are in different sectors? (unless is a name made up especially for the product like CocaCola?)

Coca cola isn't made up, it's a description. The drink originally contained coca leaf extract (cocaine) and cola nut extract.

I don't know about the trademarks though.

oops, that's true :)

what i meant is rich company that can drag you through the courts for 15 years till you're bankrupt

As far as I know Corona is a trademark and not necessarily / only a company name :)