2012 Bikes

For somebody that clearly understands maths at a high level your refusal to account for tax and NI is somewhat bewildering. You also ignore the lower rates paid to anybody under the age of 21.

http://www.direct.gov.uk/en/Employment/Employees/TheNationalMinimumWage/DG_10027201

From this page:

Current NMW rates

            There are different levels of NMW, depending on your age and whether you are an apprentice. The current rates are:

• £5.93 - the main rate for workers aged 21 and over
  
• £4.92 - the 18-20 rate
  
• £3.64 - the 16-17 rate for workers above school leaving age but under 18
  
• £2.50 - the apprentice rate, for apprentices under 19 or 19 or over and in the first year of their apprenticeship
  
  

The age at which you become entitled to the main rate was reduced from 22 to 21 on 1 October 2010. The apprentice rate was introduced on the same date.
If you are of compulsory school age you are not entitled to the NMW. Some of your other employment rights are also different.
Rates from 1 October 2011

The NMW rates are reviewed each year by the Low Pay Commission and from 1 October 2011:

• the main rate for workers aged 21 and over will increase to £6.08
  
• the 18-20 rate will increase to £4.98
  
• the 16-17 rate for workers above school leaving age but under 18 will increase to £3.68
  
• the apprentice rate, for apprentices under 19 or 19 or over and in the first year of their apprenticeship will increase to £2.60
  
  

So as of today the gross monthly wage would be £1138.56. Assuming that you were able to do this 12 months of the year it's equivalent to a gross annual wage of £13,662. Assuming a tax code for the full allowance of £7475 you would be paying 20% on the difference of £6187.72. That gives you £12,125. We've still not accounted for NI contributions. Lets work on the basic rate, again from the direct gov website:

If you're employed you pay Class 1 National Insurance contributions. The rates are:

• if you earn more than £139 a week and up to £817 a week, you pay 12 per cent of the amount you earn between £139 and £817
  
• if you earn more than £817 a week, you also pay 2 per cent of all your earnings over £817
  
  

So, £12,125/52 gives us £233 per week, take of the basic allowance and you would pay 12% on the difference of £94. So the weekly wage becomes £221 per week or a whole £887 per month.

The difference is simply not something you can ignore.