ESA Permitted Work

Hi- I am wondering if the higher limit for permitted work (£140 a week and under) is worked out as an average or week by week basis?
I get income related esa for my husband and myself as a couple. I have an opportunity to do ad- hoc work making jewellery for a very large corporation. Shifts are only five hours each time- probably once a fortnight and on a self employed freelance basis. Im just concerned with the ad hoc nature some weeks will go above the limit. I know when averaged out I wont be. Can anyone please advise? I believe its old style esa but not too sure. Thank you in advance!!!