Openreach appointment is booked tomorrow but not sure if they will turn up given the existing ONT and Fibre Point.
If the appointment is booked, they will turn up. End of story.
They likely chose to arrange an appointment because they can tell remotely that the ONT is not working.
(Presumably you ordered FTTP service through an ISP, which kicked this whole process off)
The ONT is all there but the previous owners took the power supply with them…
Which of course is why the ONT is not working - but Openreach don't know that.
If an engineer does turn up can I just get them to give me the power supply or is it worth just buying one off Amazon?
When engineer turns up, they will do whatever is necessary to fix your service to make it live.
Most likely, they will just plug in a new power supply, and check the link comes up.
They *could* decide to change the ONT - perhaps to give you a more modern one. And they might find a problem with the fibre itself, and end up fixing that.
In any case, the cost to you is zero, and the engineer will do all the work for you - so there's no point buying anything yourself.