You can earn a more per year once you reach 65 - any earnings also include state and private pensions
See the link for amounts
You must declare ANY extra income no matter how small
means-tested means Pension Credits. same rules as Income Support. you have to declare any work (paid or unpaid), and they may disregard an amount of it. (for Income support, the max disregard is �20)
The link pink gives is for your personal allowances before paying tax and is not related in any way to means tested benefits that some pensioners get.
As well as informing the inland revenue of your incomes you must declare a change in circumstances to the DSS - more coming in means a reduction in means tested benefits.