BIG warning about those credits: PLEASE do yourself a HUGE favour and make photocopies of ANY and everything you send the IR. DO NOT do any business w/them by phone and if you do make a record of the date and time you called and whom you spoke with - and hang onto it for dear life. Better yet, do everything by recorded delivery letter.
If your income as a couple if £14,000+, you are ineligible for Working Tax Credit. If they pay it to you, or overpay you, put hte money into a high-interest savings account and do NOT spend it. They can and will ask for it back - with interest.
We were overpaid b/c of their errors. They came back 2 years later and wanted £1900. Luckily we kept everything in writing and recorded delivery, but the appeals process required both an adjudicator and our MP, and took almost a year.
And we were just one of about a hundred thousand families left living below the poverty line - after tax - due to their computer's errors.
Be all means apply, but CYA!