Unfortunately, the Inland Revenue's warnings seem to be rather "the film of the book", the book being the Taxes Management Act 1970, which curiously does not seem to be at the top of any bestseller lists, and the "film", as is so often the case, only following the text of the book fairly loosely.
What the book says, roughly, is this: there is indeed a pounds 100 penalty for submitting the return after 31 January, except that the penalty cannot exceed the tax liability, so that taxpayers who owe no tax (or indeed are due a repayment) will get off paying a penalty completely.
In fact, a large number of taxpayers who complete returns have most of their income taxed at source, and their tax codes take account of any additional tax payable on investments. Even the self-employed might have nothing to worry about. "Liability" means the amount payable after taking into account any payments on account already made. If those payments are at least equal to the final tax bill for the year, again there should be no penalty.
Those of us who are particularly slack at completing returns are especially favoured. Since the Revenue will often have issued such people with punitive tax codes, denying them reliefs they should be entitled to in an attempt to provoke them to get their returns in, those people may well have overpaid tax anyway.
This could be better news than many of us realise. In the panic to get their returns in by 31 January, many taxpayers are likely to have done them wrong. If the returns are rejected, they may count as not submitted on time. So, many of those who think they got their returns in actually haven't!
Am I making all of this up? Unfortunately, the only way to check is to read through the aforementioned non-bestseller. Section 93(7) TMA 1970 is the magic talisman to quote when confronted with a penalty that should go away, along with Section 59B, which defines "liability" so helpfully.
It seems that few people are aware of this get-out, which will probably apply to a large number of the 2 million or so taxpayers who have been lax in getting in their returns. Am I suggesting the Inland Revenue have been unspeakably knavish in deliberately trying to panic the taxpaying public? As if! But the deadline has certainly been putting the frighteners on even the most relaxed of taxpayers - and unnecessarily in many cases.