HMRC are now offering an online checker tool for taxpayers wanting to check whether they qualify for the taxable grant:
https://www.tax.service.gov.uk/self-employment-support/enter-unique-taxpayer-reference
To use this you need to input your 10-digit tax reference (UTR) and then your national insurance number. Taxpayers who are eligible are then invited to log in to give up-to-date contact details.
Those whose result shows they do not qualify can challenge this if they believe this to be incorrect – I have had at least two surprise results so far for clients whose figures show they should qualify.