After the fireworks, your rocket from the Revenue

Whatever the claims made for April, January can often seem the bleakest month. The parties are over, Christmas cheer is fading and the waistline suggests a new gym membership.

Whatever the claims made for April, January can often seem the bleakest month. The parties are over, Christmas cheer is fading and the waistline suggests a new gym membership.

To top it all, there could be an unopened letter waiting for you from the Inland Revenue, for 'tis the season to file self-assessment tax returns by 31 January.

The organised among us may already have either filled in the form for the 6 April 2003 to 5 April 2004 tax year, or passed over the chore to a tax adviser or accountant. But the vast majority of us are likely to have put off the form-filling pain to the very last minute. If you are among this number, here's what you should do.

The first thing to note is that if you do not submit your tax return by 31 January 2005 you will have to pay a £100 penalty. In 2003, some 875,000 self-assessment forms were received after the 31 January deadline. IFA Promotion, the marketing body for financial advisers, estimates that fines for late returns will total around £88m this year.

Worse, the longer you delay, the harsher the fines. These include another automatic penalty for £100 in July if no forms are filled in, and other surcharges and interest payments on the amount outstanding. If you persistently fail to get details to the taxman, you could be fined up to £60 a day.

Around nine million of the UK's 29 million taxpayers have to fill in self-assessment forms. These are the people with uncollected tax to pay - typically, the self-employed, those with investment income and those with big capital gains to declare.

Most will already have received their form from the Revenue, although the onus is on individuals to notify the taxman of any untaxed profits or gains.

For example, if you've recently taken the plunge into the buy-to-let market and now receive rental income from property, you'll need to tell the taxman.

Higher-rate earners with savings income must also give details of bank and building society interest since they will have to pay extra tax.

If you have lost your form, you can request another by contacting the Revenue helpline on the number below or visiting your local tax office.

Filling in the form can be laborious so, before you start, gather all the relevant information. Since the current return is for the whole of the 2003-04 tax year, you'll need details of any income or gains and evidence of deductible expenses.

What else you have to dig out will depend on your circumstances. If you work for a company but have a separate income from a small property portfolio, you'll need forms P11D and P60 (available from your employer). These will give details of salary and other benefits to add to your own accounts of property rental income and expenditure.

Although the forms can now be navigated more easily than in the past, you may require some help in working out where the figures need to be filled in.

Helpsheets are usually included with the forms but you can use your tax office (the phone number should be on the front of your return) or the Revenue helpline for general advice.

If you have internet access, there is a lot of extra information on the Revenue's website - listed below - under the "Individuals and employees - self assessment" heading.

You can fill in your return electronically, using the Revenue's free online system. But if you have a complex return requiring additional information, it is often easier to do this by post.

However, sending in the form is not the end of the story: 31 January is also a tax payment deadline. Depending on your circumstances and whether you've under- or overpaid in previous years, the settlement will typically be the last tranche for the 2003-04 tax year (many may already have paid earlier instalments on 31 January 2004 and 31 July 2004).

To this end, online filing includes an incentive: you automatically have your tax calculations made for you.

Making sure you meet the 31 January deadline certainly won't be a highlight of your first month of 2005. But avoiding late filing penalties will at least make it less gloomy.

Contact: Inland Revenue helpline, 0845 900 0444; www.inlandrevenue.gov.uk

Francesca Lagerberg is national tax director at Smith & Williamson, the accountancy and financial advisory group. Contact 020 7637 5377 or www.smith.williamson.co.uk