You have a 18m fixed discount something like this but that does not mean the actual bill will be fixed for duration of 18m.
Example package maybe £50 and you have £25 discount. VM will apply about 6% increase based on the orginal £50.
The contract has not changed.

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M200 Oomph - 5GB SIM + 3 extra SIM family plan - BT whole Wi-Fi - Sky Q
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Don't cry over Virgin price increase instead speak to retentions team and negotiate a better deal.