Please note: I have no qualifications in Anglo-Norman, or in fact in any form of French. The following translation is intended as a fun exercise, not a rigorous scholarly undertaking, and accuracy and finesse are in no way guaranteed.
Anglo-Norman Letters and Petitions XII, p 10: J. S. Esquire to the King.
Your poor subject J. S. Esquire humbly beseeches that, being suddenly taken by J. Venour, one of the viscounts of London, and placed in custody in your local gaol and being kept there in prison, and succeeding in discovering nothing of the reason for his imprisonment save what the said viscount told the said supplicant, that he took him on the order of your gracious self, may it please your most excellent lordship in your most abundant mercy to consider that your said poor subject has been detained there for more than six weeks to the great detriment and ruin of his poor estate, and also that the said supplicant is so ill with diverse maladies that the pain will not let him live, and to send for your said supplicant to be brought before your royal person or otherwise before your most wise council to reply as justice and law demand. For God, etc.
Original text from Anglo-Norman Letters and Petitions from All Souls MS. 182, ed. M. D. Legge, Anglo-Norman Texts 3 (Oxford: Anglo-Norman Text Society, 1941). 29-30.
 The usual elaborate opening forms (“Au tresnoble/treshonouré/tresgracious” etc) are omitted. This and certain other touches give the letter an unpolished quality that make me wonder whether it was written by the prisoner himself, rather than a professional scribe. Given the circumstances, he may not have had access to a clerk.
 He does repeat himself rather here: “mys en garde en vostre countré [prison] illoeques et la demoert en prisoun”.
 Or possibly condition, health; but he turns to his health next, and “arrerissement et anientiesement” sound to me more like formal terms to be applied to one’s living, business, lands, etc, than to one’s body.