From the date of receipt of that letter you have 30 days to hand your notice in and leave penalty free.
usually the date to leave penalty free would be a cancellation date, 12 months from contract start date, so give your notice 30 days before that date.
for example if you took out a 12 month contract on March the 1st 2018, you could give your notice in Feb the 1st 2019 and when the 30 days rolls over it would be past the 12 month marker of 1/3/2018-1/3/2019
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