London's transport bosses are telling travellers on the tube system to beat the heat by carrying a bottle of water with them when they venture underground.

But how many of us are refilling our bottles with tap water rather than pouring money down the tube -- not to mention the cost of recycling the plastic bottles -- by buying a new bottle of water each day?

Cue the National Hydration Council whose eye-catching advertising campaign to encourage people to buy more "naturally sourced bottled water" -- on health grounds -- featured prominently on the underground network earlier this year.

The worrying thing for the bottled water lobby is not that people are doing what would appear to be the most sensible thing and refilling their bottles from the tap, but that Britons are replacing bottled water with sugary drinks instead.

We're told that sales of bottled water fell by 7 percent last year, with 71 percent of that decline the result of people buying sweet drinks instead. Good news for the soft drinks industry perhaps, but a worry for health officials.

Meanwhile, beneath the streets of London, the hot and flustered faces of fellow tube passengers shows just how dire it is on board the capital's underground trains when the mercury rises.

With a decent air-conditioning system on most lines a distant prospect, Transport for London (TfL) could show it cares by offering each of its cash-strapped passengers a free TfL water bottle and the opportunity to refill them at its stations.

The resignation of another key aide to Mayor Boris Johnson has sparked renewed questions over the Mayor of London’s leadership, with opposition leaders at City Hall charging that the “wheels are coming off” his new administration.

Tim Parker , the First Deputy Mayor and Chairman of Transport for London (TfL), has stepped down from both jobs, saying it was inappropriate for him to hold them as an unelected official. His resignation is the third of a key aide in the four months of Johnson’s mayorship.

Despite having a rather big job to do already in running the capital, Johnson will now chair TfL himself. In his usual combative style he announced: “Over the last few weeks, it has become increasingly apparent to both of us that the nature of the decisions that need to be taken are highly political and there is no substitute for me, as the directly elected Mayor, being in charge.There are limits, therefore, to what can be delegated.”

It begs the question why Parker was installed in both roles in the first place - the running of London’s transport system has always been a politically charged issue.

More important for the future though is whether Johnson is biting off more than he can chew by chairing TfL. Even in times of crisis, delegation to top aides will be crucial for the mayor’s success. One of the key arguments against voting for Johnson as mayor was his perceived lack of managing a large organisation, and judging by the rate at which senior aides are disappearing, he looks set for a rough ride, not least as yet another strike is looming on the London Underground.