A P60 is not mandatory.
In itself it has limited value as documentary evidence since it's issued annually and reflects the previous 12 months pay and deductions etc.
Means a P60 issued in April 2013 has pay and deduction details for the year April 2012 - April 2013
It really depends on which category of visa application and what your wanting to show.
The latest P60 wont be any help in supporting past 6 months activity, but may be helpful if going back over the past 12 months.
BTW a P60 is only issued by the employer who actually employs you on April 5th (end of the tax year)