You beat me to it DrSuperL99 and durhamlad. Allow me to go one step further with another of my patented long and boring posts.
US tax year - Jan. 2012 to Dec. 2012
UK tax year(s) - (1) April 2011 to April 2012, AND (2) April 2012 to April 2013
For a US tax return for 2012, take Jan. 2012, Feb. 2012, and Mar 2012 from UK tax year (1). Then add April 2012 to Dec. 31, 2012 from UK tax year (2). This is the reference [(2)] to your Dec. payslip.
Jan. 2013, Feb. 2013, and Mar. 2013 from UK(2) will be used next year for your 2013 US tax return.