How do you deal with an existing toxic culture?

Aside from employee happiness, a toxic culture can be detrimental to retention, attraction and the ability of your team to give their best. But how do you undo it, when it's already established? Experiences, advice and insights in conversation with Jessica Hayes (VP People & Talent at Whereby), April Williams (Founder of Can?Did!, HR & Talent Consultant), Charlotte Hamill (COO of Born Social) and Tom Fraine (Chief People Officer at Bulb).