It may be unfair, but it is perfectly legal and you will be expected to pay it. Interest on a Wonga loan will accumulate either until (A) you pay it off (B) 60 days passes after a default without any contact with them or © a contract prevents them from charging you interest. The repayment plan does fall into the final category however only once it is put into action, i.e, the first payment is completed. Also note that depending on how your repayment plan was worded, if you miss one payment, they may be able to charge you interest again, even on the period over which you were making payments. I'm not quite sure how Wonga generally handles that scenario however I know that some similar services can and do enforce similar rules.