Taking a friend whose never been to London before and would like to find a charming, quiet, reasonably-priced B&B in Central London. Any suggestions ?

What's reasonable?

I know that "rj007" recommends one that is in the suburbs. However, you might be able to get a cheap hotel using priceline.com.

about 50 GBP near Nottinghill Gate, Kensington

I am quite doubtful you will find a twin bedded room with a good breakfast in a nice B&B for that price. It will certainly not be ensuite for that price.

But check out Rick Steves' recommendations and www.londontown.com and www.venere.com and www.smoothhound.co.uk .

Priceline is a good choice if you are willing to give up breakfast.

Did you say less than 50 Pounds in Notting Hill?
Well HYDE PARK WEST HOTEL will be your only option for all criteria!
BLUE BELLS HOTEL is also near Notting Hill, but is bit expensive.
Go to www.findlondonrooms.com, and there is a special category for B&B's in London there. Or you can search by hotels by area category!

Said to be "on the fringe of Notting Hill" is Kildare Gardens found at vrbo.com/36560 or 36579.