Chris extension tubes will not work for what you want, what you need in an Extender, but this will probably lose autofocus on the Tamron, also you may have a quality issue. This is what you should be looking at and this is the cheap end of the market, such as the Canon extender costs in the region of £350-400.
http://www.parkcameras.com/12054/Ken...FcjKtAodB3QA2Q