if the job you are going for is PAYE then you will pay tax and ni as normal u then have to declare this and all self employemnt on your tax return and let the tax man tell you what you owe them.
if it is self employment, just carry on as normal
even if you get a PAYE job, you have to tell the NI that you are no longer self employed or you will receive 2 bill for ni contributions, one whilst working and one whilst self employed, and they want payment for both, i know this as i got a bill yesterday for my self employment ni for a period that i was employed phone them and told them and got told that because i never told them, i still had to pay it as they classed me as self employed
i would get a job and do the windows on the side