Jackie, you asked about the origin of 'gammon' in the sense of 'deceiving'. I have never heard, and cannot find anything about the origins of this but it occurs to me to wonder if it relates to the secret language of the Irish tinkers, Shelta, which was also called 'Gammon'. Since Shelta did serve the same purpose as cockney rhyming slang - in other words a kind of thieves' argot.

Here is a link to the subject:

http://www.christusrex.org/www1/pater/JPN-shelta.html