Savings and capital don't count for the Tax Credits benefits. It's your salary that counts.
Not quite the whole story. Clearly you have to be working sufficient hours to claim Working Tax Credits. And whilst capital, in itself, is not relevant to the calculation of WTC,
all of your income is being taken into account. Except the first £300 per year of non-working income is ignored, so cutting out the need for negligible savings income to be taken into account, in most cases.
Simple example ....... salary/wages £10000 ...... savings income £25 ....... pension income £2000 ........ so £10000 + (25 + 2000 - 300) = £11725 is the income figure that would be used in the calculation of the WTC.