The £8 credit ends in April 2020. That looks like a desperate discount you may of negotiated at the time which has been added on, separate from the 10/15 ones. That brings it to the 33 off it is showing.
However due to dates of install (or going by how your Line rental saver ended 2 months into new contract, a negotiated contract not a new install?) vs contract signing that may be a different date as that’s taking the date off your contract which seems a month later than the bill tells you.
To be clear, each bill tells you when each standard discount is going to end. That is when it will end unless you call up and negotiate etc.
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The best thing for us to do with regards to this is take a closer look in to your account.
To be able to dot his, I will need some ore information from you. I will drop you a PM asking for the details required, if you can reply to this, I will be more than happy to look in to this further for you.