Welcome to the forum!
I can think of the main options:
1- it’s possible to extract the data out of MelCloud, and estimate usage from that. I have no idea how this works with cascades, but would be happy to look into it for you. Charting flow temperatures would be useful if nothing else. Send me a private message, and I’ll explain the setup.
2- there are devices you can plug in to the heatpump to extract that same info. Again, I don’t know how this works with cascades, and may be more technical. Have a look at Monitoring & Controlling Ecodan via CN105
3- buy and install (or ask a plumber to) the heat pump monitor kit that a Robert mentions above. This will give the highest quality data and analysis, and doesn’t rely on Mitsubishi support.
For more general advice on cascades, this topic may be useful: https://community.openenergymonitor.org/t/converting-ecodan-systems-to-cascade/