By Michelle Roberts
BBC News health reporter
Walking up to the big, bright shiny bus in the middle of a busy shopping centre I wonder if I have got the right place.
The bus is visited by 100 children each month
It's a far cry from the old fashioned 'clap clinic' located down a dark alley at the back of a hospital, shrouded in secrecy and taboo.
Outside the bus there is already a group of teenagers forming a disorderly queue.
Pushing and shoving each other and generally mucking around, they look like they are waiting for a lift to school rather than queuing for free condoms.
The 4YP bus is a service for all young people in Enfield and Haringey aged between 11 and 18 who want guidance or simply someone to talk to about sex and relationship issues.
On board, the staff adopt an informal approach. Some have been teen mums themselves, others are youth workers or midwives from the local community.
The emphasis is on the youngsters to set the agenda for discussions.
As well as the settee lined reception, where group sessions take place, there is a private consultation room where youngsters can talk to a worker in confidence.
The children can get information and advice on sexual health and relationships, free condoms and a demonstration of how to use them.
From January, they will also be able to have a urine test for chlamydia - the 'pee in a pot' service.
As Briton's first and only sex education bus, it is hoped it will help reach the government's targets on teen pregnancies and sexually transmitted infections.
By 2010 the government wants pregnancy rates among under 18s to have halved.
Users can chat in groups or individually
And by 2007 it wants a 25% reduction in the number of newly acquired HIV and gonorrhoea infections.
Haringey and Enfield have particularly high teen pregnancy rates - around 73 per 1,000 compared to 50, per 1,000 elsewhere in London and 43 per 1,000 in England and Wales as a whole.
Similarly, these boroughs have seen higher rates of STIs than the rest of the country.
In Enfield between 1998 and 2002, there was a 580% increase in the prevalence of gonorrhoea compared to an average 98% increase for the rest of England.
During the same period, chlamydia increased by 130% in Enfield compared to 77% elsewhere in England.
About 100 children visit the bus each month, peaking to about 200 in the summer months.
Most of these are boys and many are from black and ethnic minority communities, figures since the project began in 2001 show.
"I come here to get these," said one 16-year-old boy waving the purple packet filled with condoms he is holding in his hand.
"I've come with him to find out what it's about and get information and stuff," says his friend, who then grins and giggles.
They tell me they found out about the service from a flyer at school.
"I've told some of my mates about this place and told them to come here," says the first boy.
"But I wouldn't tell everyone. Some people would take the piss or say are you gay or you can't get a girlfriend," he adds.
His friend tells me they received sex education at school, but that it was awkward to talk about it with teachers.
"It's embarrassing though innit. I think it's easier to ask close friends. Or to come here.
"One thing I'd never do is ask my mum."
Telsa Walker, one of the trained staff working on the bus, said many of the children they see come in with questions that they have been too embarrassed to ask friends, parents or teachers.
"Most have got their information on sex from their friends and often it's completely incorrect.
"So it's about making sure at least one peer in the group has the right information.
"Some come on the bus not knowing that vaginal discharge is normal or are confused about why their body is changing.
"Others just want to know that what they are feeling and experiencing is normal.
"We do have young people who may feel that they are pregnant or know that they are pregnant.
"Sometimes they do not like going into mainstream services. It can be really intimidating. They are scared or embarrassed.
"They know they can talk to us in confidence in a relaxed, informal and non-judgemental way."
She said one of the biggest challenges was getting the information to stick.
"With some, they will know what STIs are, but when you quiz them they won't know what the signs or symptoms are.
"Some get a bit giggly, and you have to go back over the same points, but you make allowances for that.
"We get them to set ground rules themselves before the start of discussions. Then it's easier for them to stick to. It could be 'talk one at a time' or 'no personal experiences'.
"It's completely confidential and that's one of the biggest reasons why people use 4YG."
She said rarely parents had turned up to the bus expressing concern that their children had been given free condoms.
"But as soon as they understand exactly what we are doing they calm down.
"Some come on the bus themselves to get information to give to their children about safe sex."