'But the question is - I assume - whether the notion of a practice belongs to the essence of a religion - as distinct from some other human activity. And the answer to that to
my mind is obviously no.' 

No, that isn't the question. The objection attacks a straw man.

No one is claiming that the notion of a practice belongs to the essence of a religion--AS DISTINCT FROM some other human activity. This would be the claim that there are only religious practices!

If I say that being a mammal is part of the essence
a kangaroo, it is no objection that being a mammal doesn't distinguish kangaroos from some other animals. Mammal is a genus, what differentiates the species of mammals isn't that they
are mammals, but something else.

Similarly being a system of practices is a genus, what differentiates the species of practices is something else. Medical practices aim at health. Farming aims at crops.
Religious practices aim at a relation of value to an SR.  The objection that being made of practices doesn't distinguish religion from other activities, so practices aren't essential to a religion, is no stronger that the objection that mammality can't belong to the essence of a kangaroo, since some other animals are mammals.

I wrote in 1991 (A theory of religion).

The feature of religions that provides the basis
for the theory I will present is this: a religion can
be practiced. Judaism, Christianity, Islam, Buddhism
each involve a set of activities that, when done in
the right way for the right reasons, constitute the
practice of that religion. This suggests that for
every religion, there is a system of practices that
comprises the religion. A religion is a kind of
system of practices. But plainly this is insufficient.

Medicine is a system of activities performed regularly
by doctors, but medicine is not a religion.
What differentiates practices that constitute a
religion from practices that do not? What makes a
system of practices religious? Part of the answer is
the way the system of practices is rationalized.

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