Harney has a strong City clientele, rich with hedgies, private equity principals and transatlantic links. So how does he view US tax obligations from an overseas perspective? ‘It can be a complete nightmare,’ he says, highlighting the case of Boris Johnson, who pledged to renounce his US citizenship after an IRS battle about the sale of a London property. Although many have thought about following suit, few have done so. ‘For every ten or twenty enquiries, one might actually go to the end of finalising their expatriation,’ he says, pointing to emotional links. Experienced in the UK, Ireland and the US, Harney is adept in the use of family partnerships and family investment companies as tax-efficient wealth vehicles.