I work quite frequently as an extra as its good money and have never heard of these agencies. Sometimes the ones they put in the stage are a bit dubious! The best agencies in London are Ray Knight, The Casting Collective, Mad Dog and Screenlite. Have a look for them on the web, if you can't find them i have their number somewhere. These are def the best if you can get on their books, they ring me for work maybe 4 times a week or so. Good Luck!