Sorry @Jake33 I can't offer financial advice, but the government are correct - you can't claim working tax credits if your main source of income is from property. Working tax credits is a form of support available to those who are either self employed or employed.
Generating income from property is neither - so I am confused as to why you would have applied for working tax credits if you have no form of self employment.
If this is the case I would speak to them and see if you can make a staged plan to repay the money you claimed incorrectly. @Jake33