The Revenue Online Service (ROS) will not accept a 2012 P35 Annual Return containing a negative USC amount if filed prior to Jan 28 2013.
Negative USC can occur when employees receive USC refunds.
Therefore a P35 with a negative USC amount must be filed between Jan 28 and Feb15 2013 which is the final deadline date.
It appears that you cannot print a p45 if you are unfortunate enough to have an apostrophe in your surname!
You have been warned!