TIP TOP PRODUCTION: Prime Minister Tony Blair also visited Greenwich on Monday for the topping out ceremony of the Millennium Dome - for which he predicted a long life. 'The structure has been built to last, not to be torn down after a year as the previous government envisaged,' Blair said, adding that a 'lasting use' for the Dome should be found. And he paid tribute to joint venture main contractor McAlpine-Laing for completing the near £50M structure on time and to budget. The ceremony marked the fitting of the cap to the central vent on the summit of the vast PTFE- coated glassfibre fabric roof. A gap still remains around the Blackwall Tunnel vent which passes through the Dome on the west. Contractors are said to be on course to fit a waterproof sheath around the vent by early July.