I've got this working in principle, although i'm not using the API. Instead, I'm using the method shared by @LibraSun (thanks very much for this) in this post here:
as for how this is relevant to agile octopus, I've used IFTTT to populate the data daily into google sheets (a free applet so a pro membership is not required), some sheets functions to manipulate it into a format that hubitat can extract, and then created a rule using the guidance above that runs at 2330 each night to extract the data into a series of hub variables according to the half hourly values. It seems to work nicely for the first few variables i've entered (i've not put all 48 in yet); the only thing I need to confirm is if the sheets data will update in the background without me opening it. I think it should, but haven't confirmed.
In theory I can probably also write some google sheets formulae that calculate the cheapest 1 hour, 2 hour, 3 hour window and then export that time as a further variable to use in Hubitat, in order to trigger automations at certain times.
