Yesterday, The BBC posted an interesting feature on its website which used word clouds to examine visually how the words used in political manifestos have changed over the years.
In 1945 the most popularly used word was 'must', nowadays it is 'people'.
If you are looking for the full text of older manifestos- an excellent starting point is the Political Resources website maintained by Richard Kimber which has the text of most examples from 1945 onwards
You might also like to know theat the LSE Library and archives hold printed copies. For details of their election ephemera collection and how you can help by collecting 2010 materials see the website.
For scholarly discussion of elections. See the Elections, Public Opinion and Parties Specialist Group of the Political Studies Association. It includes details of the British Election Study 2010 and contents pages from its journal. You might also like to look at the conference proceedings section of the same PSA website where you can read and download the full text of many papers from PSA annual conferences since 1994. They include many examples relating to elections and the changing nature of political communications.