The Wretched Prison Ships (Patriots in Brit Ships during RevWar)

Newsday ^ | 5-24-08 | George DeWan

[C19fan]

You are right Magwitch was not directly involved with Miss Havisham but the man he fought on the marches, Compeyson was. From wikipedia:

More than 20 years earlier, he fell in with a con man named Compeyson, the other convict he was fighting on the marshes in Pip’s childhood. Unlike him, Compeyson was a smooth talker who could pass off as a gentleman.

At the time Magwitch met Compeyson, Compeyson had another confederate named Arthur who was deathly ill. Arthur and Compeyson “had been in a bad thing with a rich lady (who turns out to be miss Havisham) some years afore, and they’d made a pot of money by it.” Now, Arthur was dying and going insane too, although Compeyson showed little concern. Arthur dies after hallucinating his late wife taking him to the afterlife.

Magwitch realises he should have taken warning from the example of Arthur about Compeyson’s perfidy but did not and was betrayed as a result. The two had several misdemeanour brushes with the law, but after four or five years were brought up on a felony charge of passing stolen notes. Compeyson set up Magwitch to take the greater part of blame by telling him they would put up separate defences. As a result, at trial, Compeyson passed off as the dignified gentleman, while Magwitch had to sell his belongings to hire Jaggers. Magwitch received a fourteen-year sentence while Compeyson was given half of the time.

They were put in the same prison-ship, but Magwitch could not get at him. At one point he did get ahold of Compeyson but was immediately seen and placed in the “black-hole” of the ship, from which he promptly escaped and made his way to shore, where he was hiding among the graves when he encountered the then-seven-year-old Pip. Young Pip’s mention to Magwitch of the other person he encountered on the marshes made Magwitch realise Compeyson was there, too, apparently driven to escape by his terror of Magwitch. Magwitch attacked and beat Compeyson until he was stopped by the arrival of the soldiers. Compeyson was again given a light punishment for his escape, but Magwitch was retried and sent for life imprisonment, though he later was released on condition of never returning to England.