The earl and the pickpocket - Helen Dickson

Found out!

Heloise Edwina Marchant longs for the beauty and comfort of her former life, before she was forced to flee her family home. Coming to London in the guise of a boy, she has learned the hard way how to survive among the hovels and alleyways of St. Giles. There is shame in having to pick the pockets of unsuspecting passersby, and the inevitable happens-she is caught!

The gentleman who seizes hold of her is not angry for long. In fact, his firm kindness is almost her undoing. For he has come to St. Giles with a purpose-and she will help him if she doesn't want to be reported to the authorities. But how can she agree, when at any moment this good-looking man could find out that he is a she?