Just posted this link on another thread, and thought it was worth posting out for everyone to read. Please know that I don't know all the ins and outs, but I do know that this information isn't commonly known!
When calculating your income for the year, tax credit now often takes the figures from your personal tax information. However, this might not be right, and you could take some off your amount earns, meaning higher payments.
If you're on maternity/paternity/adoption leave, the first £100 a week is 'free' and can be taken off you gross earnings. Any professional subscriptions, indemnity insurance, work costs (but not commuting) can also come off. Redundancy pay under £30,000 doesn't need to be declared as income. There are others on the list, which can be found here
I wanted to mention this, as it could take thousands off someone's income, and for us low earners, can make a massive difference to our weekly income.