I can claim up to £300/week up to 80% of my costs.
Yes and No. The max claim amount is "80% of £300".
So if your childcare costs are £300 or more, then subject to you qualifying for Childcare Element of Working Tax Credit, you can get £240 towards the cost.
See WTC2 (pdf) and WTC5 (pdf)
Also of interest is this from WTC5
If you employ someone as a registered or approved home childcare provider, for example
a nanny, you can get help for up to 80 per cent of the gross costs of employing them ?
within the limits opposite. You can include:
? the costs of any employer?s National Insurance contributions you pay
? the costs of any benefits in kind you give and
? any other costs linked with employing that person.
So you CAN include the Employers NI within your childcare costs figure, if needed.
With more than 2 children the amount of tax credit does not increase... though that's not such a problem with nannies, as a nanny is not paid per child, unlike other forms of childcare.