A couple of days ago my daughter and I checked out a Vietnamese noodle bar called Pho, conveniently near Oxford Circus. It specialises in Pho (pronounced fuh), an unbelievably addictive, tasty noodle soup, not flattered by the rather murky picture to the right.
The basic broth, which usually includes some kind of meat, is quite bland - you zip it up to your own taste with fresh herbs such as coriander and mint, chillies and/or chilli sauce and fish sauce. I thought they were slightly stingy about the herbs. I haven't been to Vietnam but have eaten a similar soup in Bangkok where you got a great jugful on the table. But it's tasty and, as they correctly point out, very healthy.
My Pho Tai Bo Vien which also included meatballs was £7.45, not bad for a West End lunch though probably 10 times the price you would pay in Vietnam. I suspect you could also find it more cheaply in the Vietnamese restaurants along Kingsland Road in Hackney though I haven't been down there for a while.
I've also had a go at making it at home with reasonable success - you can find the recipe on my student website www.beyondbakedbeans.com.