I think it is a power because it states "in default of that appointment" implying that the first element is not compulsory (i.e. the trustees do not have to carry it out).
With a discretionary trust, the property subject to the trust does have to be distributed amongst a certain class, but the trustees can choose individuals from that class to benefit.
Additionally, look at the given postulant test for the power (Re Gukainbain's (sp?) Settlement Trusts) , as it may not be possible to identify an individual as a member of the middle classes (e.g. what does it mean to be middle class?) as if it is not the power would fail and it would become a gift over in default.
Also, I think the power may also fail for capirousness (can't remember the authority) and or administrative unworkability (ex.p. Yorkshire something council) because (a) the settlor has no link to the middle classes of Knotty Ash and (b) there may be too many individuals who could be considered 'middle class in knotty ash' for the trust to work.
I haven't done this for a while so I am a bit hazy, but good luck in the exams.