Should I get a macro?

Couldn't be bothered to trail the Nikon round Glasgow in the rain so took this lazy option and concentrated on shopping.

Popped into Jessops to get some idiot-friendly advice on macro lenses. I've got my 18-55mm and my 55-300mm. Even with them, I am lazy about learning so ......

-- do I need / should I get a macro?
-- if yes, where on the price range should I alight? There was one at £350 the guy described as 'lovely' !!! and another at nearly £700 ... what produces the big differential?

Answers in idiot-speak on a postcard please.