How does the number of hours you work impact on allowances - 14 - 16 hrs week?

[Tigi]

At the moment I work 14 hours a week, officially. I now do some overtime which takes it over 14 hours. Does this affect my tax credits? I get £41.70 paid to me every 4 weeks, working tax? plus child allowance. I know my salary won't be above the bandings they send me from the tax office, you know the 'tell us if you earn below x or above x pounds'- is that the tax office?
My thinking is that if i increase my hours to 'set' rather than 'overtime' my pension, and hol allowance will increase. Just wondered if I gain or lose anything with 14 or 16 hours?