Hi
I have recently just finished Incapacity Benefit, and I was doing the same as you working too. As IB is taxable the secondary income - e.g. your job is the one which you will have to pay tax on whether your employer likes it or not. As Taylortax says you must tell your tax office immediately your situation. Be warned though the DWP who pay your IB took four months to advise the tax office I was getting IB and working therefore my second wage was taxable. When I finished IB and the tax office sent my new tax code, because the DWP took so long to inform the tax office I was also working, they said I owed �500 tax , luckily for me I didn't.