Can someone explain tax and child credits

[2014isagoodyear]

I am paid tax and child credits each month. My work is now paying me extra money because they are taking away my pension so I am getting a lump sum paid into my wages, I phoned the tax credit line and they have sent me a revised claim and it's only going down by about £20 a month this year but it said the estimated monthly payment next year will be 120 less. Can this be right? My work are only paying me £1000 extra?
Thanks

You can check the award either on the HMRC website or the benefits checker at www.turn2us.org.uk. What you'll probably find is that your extra £1000 has taken you over a particular threshold.

Thanks
Thats what I thought might happen. I've had a look on the web sites but was wondering if anyone knows if it better to earn less. I get £15700 and adding the extra £1000 on it looks like they are going to take it all back next year by giving me £100 less a month?
I thought it was real time and if you informed them it would not effect next years claim.

Earning the extra could possibly be taking you over the threshold for working tax credits, meaning you will lose those and just get child tax credits.