For the last couple of years I've been attending Marta's Sunday morning yoga classes. I really enjoy them - Marta is friendly, a good teacher, and she pays close attention to everyone's positions and postures. These are quite dynamic classes, I definitely feel stronger for doing them. So I'd highly recommend them.
They are every Sunday morning, 10am-11.15am upstairs at the TCCA on Green Lanes - check out her facebook page www.facebook.com/omlightyoga/