...National Insurance Contributions...

National Insurance Contributions (NIC) are normally payable by the self employed and employed. For year commencing 6th April 2007 :-

Employed - Class 1

Employees Employers
  Weekly Monthly Annual   Weekly Monthly Annual
0% £100 £433 £5,200 0% £100 £433 £5,200
11% £670 £2,903 £34,840 12.8% <--- Balance --->
1% <--- Balance --->  

There is no upper limit on employees and employers NIC, however note the lower rate for employees.

Self Employed

Class 2 Flat rate of £2.20 per week if earnings over £4,635
Class 3 Voluntary £7.80 per week
Class 4 8% on profits between £5,225 and £34,840
1% on profits in excess of £34,840

Class 2 can be paid by monthly direct debit or by quarterly cheque.
Class 4 is collected as part of self assessment tax liability.

NB. NIC is not payable for people under 16 years or over 60 years of age for women and 65 years for men.