Do payments into an RBS Retirement Savings Plan count as a deduction for Tax Credits?

Hi there

I am about to start a new job with RBS and am now considering paying into the Retirement Savings Plan.

My question is: if I pay money into the Retirement Savings Plan, can that money be deducted from my income for Working Tax Credits purposes? E.g. I my earnings are 15000/year and I contribute 100/month into the savings plan, can I declare an annual income of 15000-(12*100)=13800? Or do they still take my income as 15000?

Thanks!