In 2011, I have been doing some voluntary work and from time to time company paid me for that and usually they have been taking tax from it but last pay about £600 they did not as I have been taken of their books. At that time I have been also working part time as head barman so my wages where not that big. So I believe I should pay some tax but have no idea how and what.
If I understand you correctly you were at one time an employee of the voluntary organisation to which you refer. If that organisation informed you that you were “off their books” then I assume that they gave you a form P45 at some stage and that you gave that form to a subsequent employer.
In those circumstances if the voluntary organisation made any payments to you after having sent you a form P45 without deduction of tax I think you should report that fact direct to HMRC to clearly indicate that you are not trying to evade tax but merely want to ensure that your tax affairs are in order.