The source also said: "Sienna also enjoys singing and has dreamed of one day being a rock star.

"Rhys has been encouraging her to do something about it."

The aspiring musicians have apparently been rehearsing their music while enjoying the sun in Mexico City.

Sienna brand bottom of luxury list - Nov 30 2007
A luxury brand promoted by Sienna Miller has been criticised in a WWF-UK report.

Tod’s came bottom of the conservation charity’s ranking of 10 luxury groups assessed for their environmental and social performances.

WWF-UK’s Deeper Luxury report warns that Tod’s could be "a liability" to Sienna’s reputation.

The charity didn’t criticise the actress for endorsing the shoe and leather brand.

But it called on celebrities not to back luxury brands from firms with poor environmental and social track records.

WWF-UK launched a "star charter" which sets out six factors for famous faces to consider before they back top labels.

The charity wants celebrities to "recognise their potential to influence consumer behaviour" and seek expert advice about firms before supporting them.

The scores were based on how the firms were judged both by the media and by other NGOs, and secondly via analysis of the firms’ own sustainability reports.

Tod’s features pictures of Sienna Miller on its website.

"Tod’s may represent a liability to Sienna Miller’s reputation," the report says.

WWF-UK senior policy adviser and report co-author Anthony Kleanthous said: "We are not saying Sienna Miller shouldn’t be endorsing Tod’s. We are saying celebrities are brands just as much as luxury companies are brands.

"We think it is important both for the endorser and for the brand to be considering the environmental and social aspects of everything they do."

Tod’s got the lowest overall score at 34.9.

Sienna no-show at Otis trial - Nov 28 2007
"Critical witness" Sienna Miller did not show up for Otis Ferry’s trial after sending a text to say she could not make it.

Otis is accused of preventing photographers taking pictures of the actress and his brother Isaac when they left a London nightclub together in February.

The son of Roxy Music singer Bryan Ferry allegedly leaned through the car window of two photographers, grabbed their keys and threw them away.

Phillip Lucas for the defence told the judge at West London Magistrates’ Court that information from the press suggested Sienna was in Mexico.

He told the court: "I have a witness difficulty in that the actress Sienna Miller is not here.

"I’m going to explore the possibility of having a statement read."

He said he had to consider whether or not to make an application to adjourn, adding the information he had was that Sienna would not be able to attend the trial.

"She is a critical witness from our perspective," he told the court.

District Judge David Simpson, who is hearing Ferry’s trial, said: "I think it is too late in the day to have an adjournment outright so if anything we are going to be part-heard."

Otis, 25, denies two counts of criminal damage and the trial is due to be opened in front of the judge later today.