HMRC's online filing website pages mention Extra-statutory Concession B46 which states: 'The Revenue do not propose to charge a penalty in the case of ... returns received in their hands on or before the last business day within seven days following the statutory filing date' (i.e. 19 May).
They say that this concession allows for postal delays and problems with Internet connections, including delays and occasional slow service experienced.
HMRC specify that they will not charge a 2007-08 late filing penalty if they have the return by midnight on 26 May 2008. (26 May is the spring bank holiday, but this is the date provided on HMRC's website.)
Small employers will still get their tax-free payment if they send their return online after that date, but may be charged the late filing penalty.