Boris Johnson has claimed that he made a “catastrophic” mistake by apologising for partygate. The former Prime Minister, who received a fine alongside then-Chancellor Rishi Sunak during the Metropolitan Police’s Operation Hillman investigation, was forced out of No10 after support tumbled following the revelations. Writing for the Daily Mail ahead of the release of his tell-all memoir ‘Unleashed’, Johnson said: “Without belabouring this weary business, I think I made several catastrophic mistakes in the handling of the story. “I should have been far more robust at the outset. I tried to defuse…