The contract is one I have taken out on behalf of my son, the monthly bill was previously £21.70 per month however in September I added 2 data add-on's (one for £2.50, one for £6.67) and later upgraded the data allowance, increasing the monthly bill to £29.99. I have then been charged £110 for the period 11 sept- 10 Oct and a further £73.39 for the period 17 sept-10 Nov (which you'll note, overlaps significantly) 

Please could you explain why I have been charged so much??