This is a very important topic. Historically, it has been heavily examined by Edexcel. For example, on the June 2013 Core 3 paper there were two questions on Functions accounting for 18 of the 75 marks on the paper. Further down this web page you completed Functions questions from 2008-10 (blog entry dated 21 Nov 18). The questions supplied here are taken from 2011-13 papers.
Here is a Functions Question with the mark scheme, together with a video solution:
Functions Example (7:54)
Further Practice: