Decode Elster Merlin 868 Water Meter

I have a task to receive data via LoRa from some Elster Merlin 868 meter readers.

The device has joined our local Lora network and that is pushing the received payload to an MQTT broker.

In turn, I have a simple NodeRed flow that receives the payload and displays it in a debug node.

So, we have communications and I have the received payload, which is a long HEXA value but I'm at a bit of a loss what to do next.

I believe that it's encrypted and I do have the key for it. Once decrypted, it will need to then be appropriately parsed.

Does anyone have any pointers / reference material that I can refer to so that I can get a foothold on this?

Ultimately we will move over to a commercial package for this job, as it will need to scale significantly, but this is just a proof-of-concept thing for now.

Thanks