Companies House have been leveling automatic penalties for late filing of accounts since 1992 and for the year 2010/2011 over £99.4 million was collected by them in penalties!
When you are filing the first set of accounts for your company and they cover a period of more than 12 months, they must be delivered to Companies House within 21 months of the date of incorporation if a private company and within 18 months if a public company, or, if longer, within three months of the accounting reference date.
In the second and subsequent years a private company has 9 months from the end of the accounting reference period and a public company has 6 months in which to file.
You should make sure your accounts are filed at least a few weeks before the deadline where possible in case there is an error and they are rejected and need to be corrected and re-filed.
For a private company the filing penalty starts at £150 for accounts which are not more than one month overdue rising to £375 for up to three months over, to £750 for up to six months and £1,500 for over six months overdue. For public companies, the penalties are much more severe, starting at £750 and rising to £1,500, £3,000 and finally £7,500 for accounts filed more than 6 months overdue.
Companies House does offer a free eReminder service for accounts and annual returns.
To activate this for your company you need to log into Companies House WebFiling at
https://ewf.companieshouse.gov.uk//seclogin?tc=1 and select 'Join eReminders' from the company overview screen and enter your email address.