Who writes Pip a letter?

Please do not read this answer if you haven't finished Great Expectations by Charles Dickens yet because it contains spoilers. Pip believes at first that Miss Havisham sent him to London but later finds out it was actually Abel Magwitch, the convict who Pip helped. Pip was disappointed by this information because he desperately wanted to be with Estella and thought that Miss Havisham approved of him enough to send him to London.