the charity i work 12 hours a week for has asked me to increase my hours to 18 hours a week. However, at present i am under the tax and ni threshold so they would prefer me to go self-employed. Do you see any problems if i really only have one contract ? I would then be earning enough to go over tax, ni, thresholds, so would it be very complicated to work out amounts?
You would have great difficulty in convincing the Revenue and Customs that you were self employed. I work in the construction industry and our sub contractors have to follow stringent rules to qualify as self employed. The circumstances you outline would not be sufficient in my opinion. Also as you say it would be very complicated for you to sort out. You should remain as an employee.