From 1st April 2015 you only pay contributions on the pay you actually earn. Each April, your employer will decide your appropriate rate of contributions for each employment by matching your actual pensionable pay to the appropriate band in the contributions table.
Kate works part time. She works half the hours of a full time colleague.
If she were a full time employee her pay would be £25,000 per year, with an employee contribution rate of 5.8%. As she only works half of this time her actual part time pay is £12,500 so this makes her contribution rate 5.5%.
See "how are my benefits worked out?" for more details about how your benefits are worked out when you retire if you work part time.