Heh, heh. I like it.
Because it appears to be asserted as fact in the various tax return notes, where it says nothing about it being conditional, as JB has just suggested, upon the relevant taxpayer having received a tax return.
It could be that the TRN authors are making the assumption that the fact that a taxpayer is reading the notes means he has been sent a tax return. That is however a dodgy assumption, since it's possible that he has simply seen them by browsing online or seeing a friend's, or by seeing his own of an earlier year, but is not being sent one for the relevant year.
If JB (and Which?) are right, then IR/HMRC are guilty of not telling the whole truth in their notes, to the extent of possibly misleading someone into thinking they must notify when they need not.
If JB is right, and I'm happy to bow to his superior knowledge, it will also cause me to withdraw my comment about non-enforceability on the grounds that the taxpayer could easily be innocently unaware of the reporting requirement.