So a bit of a status update:
A buddy pointed me to the following page Input slider values to a python script for RGB LED - #14 by nnelg which actually helped me a lot to setup the sliders to work correctly.
So now I can actually update the volume within HA 
So you need to create some entries in a shell_command.yaml (if you don’t have it create is and don’t forget to add the include line in your configuration (shell_command: !include shell_command.yaml))
In there I added the following entry:
zone1_volume_slider:
python3 /config/scripts/htd-lync12.py volume 1 {{volumevalue}}
Then I created a numeric helper
Then I created an automation
alias: HTD_Kitchen_Volume
description: Automation to handle the volume slider for the HTD Zone 1 (kitchen/Livingroom)
trigger:
- platform: state
entity_id: input_number.kitchen_livingroom_volume
condition: []
action:
- service: shell_command.zone1_volume_slider
data_template:
volumevalue: '{{states.input_number.kitchen_livingroom_volume.state | int}}'
mode: single
After adding that so my lovelace card and testing it out by moving it around it successfully changed the volume (up and down).
The HTD only supports a max volume setting of 60 so I made sure my slider was set the same.
Now my speaker card looks like this:
Another side-note, which you’re probably aware of, changing the volume when the speaker is off powers it on, this isn’t the script it’s the way the HTD system works.