Detectives hunting missing eight-year-old Sarah Payne are investigating a possible sighting of the girl with a man at a service station.

But plans to study CCTV video footage have been dashed after the service station revealed security cameras were not working.

A child fitting Sarah's description and said to be upset, was seen by a woman in the ladies' toilets at Knutsford service station, on the northbound carriageway of the M6 in Cheshire, assistant chief constable Nigel Yeo told a Press conference at Littlehampton police station.

He said: "She was seen with a man described as white, aged 30, about 6ft tall, with a solid build, crinkly sandy hair, clean shaven, and with a strong jaw and a wide nose."

The man was wearing washed-out black jeans, a dark grey bomber jacket and a plum shirt.

The middle-aged woman asked the girl what her name was and she replied "Sarah". But the woman, who would have had no idea a child had gone missing in Sussex, took no further action when she watched the girl walk off with a man who had been waiting outside the toilets, believing he must be her father.

Mr Yeo said the child was spotted at 5am on Sunday, July 2, the morning after the eight-year-old vanished from a secluded country lane in Kingston, near Ferring.

"It's very important that if that person with the little girl is aware that they were there we really need to get hold of him," said Mr Yeo.

"We want him to come forward to eliminate him, or otherwise, from this inquiry. Until we have a reason to stop going into it we will continue to look at it."

Detective Superintendent Alan Ladley, who is leading the hunt, said officers had planned to examine CCTV security camera footage from the service station before being told the cameras were not working.

Police now hope the pair may have visited other service stations in the area.

Earlier, Sarah's mother Sara and father Michael made another emotional appeal for help, surrounded by some of the many goodwill messages the family has received from all over the country.

Clutching a white teddy bear, Mrs Payne was close to tears as she said: "This is for you darling. They (Sarah's brothers and sister) never stop thinking about you, darling."

The fresh plea came as it emerged detectives working on the search are to be offered counselling because of the distressing nature of the hunt.

Sarah's family have already been offered specialist counselling.

Her parents said the belief that she was still alive was driving them on.

"That's what is keeping us going, that she's out there somewhere and we are just waiting to find her," Mr Payne said.

"There is no question in my mind, it is just a matter of time and, yes, it does get harder."

His wife added: "Every day gets harder than before. In fact, every hour gets harder than the one before."

The couple said it was particularly difficult time for their younger daughter, Charlotte.

"They are extremely close. They always go to bed together so obviously bedtimes are very hard for Charlotte. It's a terrible time for her.

"She talks about her a lot. Whenever Charlotte has something to eat she takes me to one side and says, 'I think Sarah will be hungry mummy, do you think Sarah will be hungry, do you think Sarah needs a cuddle?'"