Kath - did the Tax Credit people already know about the DLA? If they did, then additional tax credits should have been paid anyway, regardless of Carers Allowance. If that is the situation, getting Carers Allowance would not add to the tax credits. In other words, it is the entitlement to DLA which triggers the extra tax credit, not the entitlement to Carers Allowance.
Carers Allowance is taxable so has to be declared to the tax credit people on the annual declaration each year, & will count as income in calculating the total amount of tax credit payable.