Don't panic, but you've only got three days left. If you're still waiting for that burst of enthusiasm for fiscal affairs to hit you, it's too late.

Inspired or not, self-assessment tax returns must be in by Tuesday if you want the Inland Revenue to work out your tax liability. But if you calculate it yourself you have until the end of January. And there's no shortage of willing helpers offering to make the job easier - for a price.

The Revenue offers free help. But its watchdog, the Adjudicator, said this month some tax officials "continue to have difficulty looking at problems from the taxpayer's point of view". Getting an accountant to do your sums and give you advice could give you more than breathing space. It could save you money.

John Whiting, head of personal tax at accountants Price Waterhouse, says this is what makes qualified tax advisers better value than tax form completion services: "You might be able to spot things yourself, but getting your allowances and getting as many tax deductions as you can for your tax expenses, that's our value-added."

For instance, one client made a big capital gain in one tax year, on which he would be liable for capital gains tax, Mr Whiting says. The firm pointed out there was reinvestment relief available, which would let him defer the capital gain if he wanted to reinvest in something else.

So what sort of person needs a tax adviser? Anyone who comes into the category for self-assessment, according to Gay Hinton of the Institute of Chartered Accountants in England and Wales.

"The only people who don't need one are people who get all their income from one source, for example people on PAYE," she says. But anyone with income from shares or property could benefit from an accountant's advice, she says.

The new self-assessment system has worried many people. But hype could be to blame. "There has been a certain amount of scare tactics, undoubtedly," says Mr Whiting. "It has got a lot of people concerned and that is making a few more think about seeing a tax adviser."

Do you have to be a maths whizz to work out your own tax? "I don't think it's difficult at all because the booklet tells you everything you need to do," says Claire Merrills at the Revenue. This is the green Tax Calculation Guide booklet which comes with your return.

But for some, the new system presents problems. "It is true that some people, particularly those on low incomes or those who have language problems, will find the forms a bit more difficult," says David Brodie, director of charity TaxAid, which gives tax advice to those on low incomes.

Even if you do miss the Tuesday deadline, paying for help or doing your own sums are not the only options. The Revenue says it will still calculate your tax for you past 30 September, though it can't guarantee it will be able to get a statement, which says how much tax you owe or are owed, back to you by 31 January. This is the deadline for paying any tax you owe.

If this happens, taxpayers could over-estimate tax they owe, or pay the amount they paid on last year's tax return to make sure they won't be fined. The Revenue's helpline will be available for the rest of the year to aid those doing their own sums.

How much you have to pay an accountant varies enormously. For advice as well as completing your tax return, fees would be at least pounds 100 to pounds 200. If you've got assets, the bill could come to thousands of pounds. Negotiate before work begins, the ICAEW says. And make sure the adviser is qualified.

Some accountancy firms offer a special low-cost service for completing your tax return. For a basic fee of pounds 69.95, SimpliTax completes your return and calculates your tax liability. More complicated tax affairs mean a higher fee. But it is important to realise this type of service does not offer advice. "The person who's helping you is not a tax expert - they're just trained to fill in tax forms," says Mr Brodie.

The computer literate could turn to a tax return software package. The Consumers' Association sells its TaxCalc software at pounds 29.99 for non-members (pounds 24.99 for members). Its on-screen interview takes you through the form, which you print at the end and send. It rephrases questions from the Revenue form at the bottom of the screen, and this can be really helpful for some of the more arcane tax office lines.

Intuit's QuickTax is similar to TaxCalc. It comes as part of personal finance package Quicken Deluxe which sells for pounds 59.95. As a separate programme QuickTax is available from Intuit on floppy disk for pounds 19.95. Its complex graphics mean the programme is slow on older computers. An error in the QuickTax software was discovered last month but Intuit is sending customers an updated version of the programme from the beginning of October.

Whatever happens, don't miss the deadlines or you could be fined. If you haven't received a self-assessment tax form and are a higher-rate taxpayer, self-employed or have complex tax affairs, you should contact your tax office by 5 October. If you don't, but you do owe tax, you could be fined.

ICAEW, 0171-920 8100 (to find a chartered accountant in your area ask for the General Practitioner Board); Inland Revenue helpline, 0645 000444; SimpliTax, 0645 450550; TaxCalc, 0800 252100; QuickTax, 0800 585058; TaxAid, 0171-624 3768 (9-11am weekdays)