Olympic Ambassadors will be back on the streets of London today. Volunteers will be in "pods" at St Pancras International Station to welcome tourists arriving on the Eurostar.
It's the first of a series of pilots to test the project. There are 55 volunteers taking part.
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.