If you pay your VAT late to HMRC, even one day late, your card will be marked for a VAT penalty called a 'default surcharge'. The first late payment doesn't attract a monetary penalty, but the second occasion on which you are late within 12 months triggers a penalty of 2% of the VAT due. The third, fourth, and fifth occasions of lateness increase the percentage of the penalty to 5%, 10% then 15% of the VAT due (ouch!).
You may not notice the first two penalties set at 2% and 5% of the VAT due as HMRC will only demand payment from a small business if the total penalty amounts to over £400. However, you will receive a warning letter, and you should appeal against the penalty if you had a reasonable excuse for paying late.
Not having the money available to pay your VAT bill is not a reasonable excuse. If your business has a cash flow problem you need to ask the HMRC business support service for time to pay before the VAT becomes payable, or we can do this on your behalf. The number to ring is: 0300 200 3835, and it's open every day. Don't ring the VAT helpline as they can't deal with VAT debt issues.
If your VAT payment was delayed by circumstances outside your control, for example a computer failure at your bank, that would be a reasonable excuse. However, you do need to present evidence of this reason when asking HMRC to review the penalty. Around 60% of VAT penalties are overturned on review, so it's worth a try!