Catherine,
It is perfectly permissable for any individual to file their tax on line. Indeed, the Taxman has attempted to simplify things for 2007/08. Although, there are no financial inducements from HMCE to register for Income Tax self Assessment, there is a kind of extended time limit for the 07/08 return to be filed with HMCE in that, for paper based returns the filing deadline has been brought back to 31 October 2008, although you still have until 31 January 2009 to pay any tax due. The filing deadline for electronic returns is still the 31 January 2009.

I would always recommend that a professional adviser is used when you have either income other than under PAYE, have made a capital gain or loss in the year or if you are unsure about the deductions available to you. I hope that this helps.

Links:
www.hmrc.gov.uk/sachanges/