It all depends on how much pension you get and how much the job pays.
Basically you would normally be entitled to a tax free pay allowance of £8109 this year. This is probably currently set against your pension (see your payslip), so any amount above this would be taxable at 20% up to £34,370 per year, with higher tax after £150,000 per year. Usually any second income would initially be taxed at 20% on the whole lot, as you have had your tax free allowance on your pension.
However, if your pension is less than £8109, you can have the difference set against the income from your job. This would need to be done by HMRC, who would issue another tax code to your employer.
I work in pension payroll so this comes up all the time - I would recommend talking to the tax man, even now, because they can give you figures based on your current situation