Sometimes the most useful information surfaces in a surprising way.

Amidst the swirl of reshuffle speculation, a conversation overheard on the 16:43 from Chichester to London Victoria has provided something more concrete.

Well, I say overheard - by all accounts the very loud phone call was more of a broadcast of confidential and very sensitive information.

This is the account posted the next day on Facebook.

Ian plainly refers to Secretary of State Iain Duncan Smith. Esther would be Esther McVey, the MP and former TV presenter tipped for a top job in the re-shuffle - which it seems is now imminent.

The facts fit the timings. IDS was indeed in Portsmouth that afternoon, meeting Portsmouth South Conservative Flick Drummond at the "allAboutTea" shop on his way back from a speech to the Local Government Association in Bournemouth. But the other convincing factor is the listener's naivete about politics.

The person listening was Sarah Quinney, a 35-year-old stage manager who says she spends much too much time on trains, and has no political interest, but knew this conversation was different.

"She was almost like a teenager showing off, look at me - the Big "I am" - so loud and posh, and the end of every sentence was inflected. It was very annoying."

Sarah Quinney says she was sitting right behind the woman, who was talking to a man though she couldn't hear his end of the conversation.

"The call kept dropping out because of the tunnels but she rang him straight back, talking about Ian and Esther. She didn't seem to care. She said 'Ian wants to go and has agreed to go, he's been fed up for a while waiting for decisions from the Treasury'."

"She said someone called Esther wants to take over but is worried that she's been too much of a bitch, p***ed too many people off.

"She said she'd just come from meeting Flick Drummond in Portsmouth - who was described by the caller as 'not very savvy'."

The Conservative candidate for Portsmouth South Flick Drummond today confirmed she had met Ian Duncan Smith and his team, but said she herself had no knowledge of the re-shuffle.

"All I can say is that people should be more careful when they are speaking on the phone in public places," she added.

Friends of Mr Duncan Smith - responding to separate rumours that he is to be moved from the DWP - have told BBC News he is going nowhere.

Romilly Dennys, a DWP special adviser, is at the centre of speculation that she was the woman on the train talking about the reshuffle.

"It is just complete nonsense... some person on a train has heard two names and managed to cobble together a story," she said.

"I am pretty convinced she (Quinney) is a Labour supporter.

"The whole thing is just a complete storm in a teacup. Someone overheard two names and she just created this whole story. The story is completely incorrect".