When does the ratio of a judgment have no precedential value?

It is well established that cases as such do not bind, their rationes decidendi do. However, the rule of precedent does not apply to decisions given per incuram (which are manifestly wrong) or in relation to mere assumptions without argument (where part of the ratio of an earlier decision had been assumed to be correct by the earlier court but was not the subject of argument before, or consideration by, that court). The recent Court of Appeal judgment in Mercy Global Consult Ltd illustrates how rare it is for those exceptions to be engaged in practice, with the court finding that the ruling in Mainpay was binding on it – and as a consequence, holding that there was a distinction for VAT purposes between the supplies of staff and the supplies of services provided by those staff, and that it was wrong to conflate them.