If you have had a primary school appeal with Lewisham in 2012 I strongly suggest that you contact them to confirm that they have not broken the data protection act by sending a copy of your letter to another parent.
Please be prepared for them to initially deny this; please ask them to re-check to be 100%, this was done to us but finally they accepted a breach.
This is a very serious situation as most appeals will contain very sensitive information about you and your child and this in itself may be grounds for a new appeal.
Due to the legal nature of this and the apparent ongoing investigation I am unable to disclose details of those I know have been affected but if I can help with anything else please send me a PM. The best email address to use it firstname.lastname@example.org