I very helpful, easy to understand, and concise paper has been put together on how to separate Lesser and Mealy Redpoll.
Plenty of images used throughout of typical individuals as well as the trickier birds.
The paper is published by Lee Evans and can be found here:
Here is a list of other Repoll literature which deals with the separation of Lesser and Mealy Redpoll:
http://www.worcesterbirding.co.uk/redpolls_58.html - Looks good, haven't read it yet as of updating this post.
http://birdguides.com/webzine/article.asp?a=2405 - For thoose who subscrbe to BirdGuides.
Until next time, Foghorn out!