The message from HMRC says:
If self-assessment customers claim or have claimed tax credits and reported an estimated income figure on their tax credits renewal in 2017, they now need to tell HMRC of their actual income by 31st January.
Customers cannot currently notify us of their actual income for the previous year; they can only do this by phoning our helpline on 0345 300 3900.
There is help available to work out the correct tax credits income by going to www.gov.uk . Search for ‘tax credits: income working sheet’ (TC825).
Self-assessment customers must provide their net profit figure(s) from their 2016/2017 tax return. If they also have other income that was previously reported as an estimate (for example, dividends, investment income or interest). They must also tell us these actual income figures by 31st January; they need to do this as soon as possible to make sure they get the right money.
If you have queries on this, please call the helpline on 0345 300 3900.