I might be wrong but as far as I am aware, class 2 contributions are nothing to worry about, just £13 or so a month Direct Debit. Your Class 4 contributions are calculated on your taxable income after expenses, I think at around 8%.
IR35 Regulations will determine your 'employment status', but if you are working on a site, you shouldn't be affected as this is the normal setup.
So all that will happen is that you will work, invoice, get paid minus 20% then do a self assessment tax return to adjust the difference between what you have paid in tax and NI, and what you should have paid which usually works in your favour once you take into account the tax you have paid on untaxable income (allowance) and expenses.
This tax return is due AFTER the end of the tax year, so if you have just gone self-employed, you won't have to do anything until after 5th April 2014.
if you choose to pay an accountant to do your self-assessment, don't pay more than £250 all in.