What's wrong - Template to fetch an MQTT timestamp value - not working

Configuration
1

I have the following esphome to send a time stamp when the voltage update happens.

# Voltage Sensor  
sensor:  
  - platform: adc  
    pin: GPIO1  
    accuracy_decimals: 4
    name: "Battery voltage"
    attenuation: auto  
    update_interval: 10s
    on_value: 
      - mqtt.publish: 
          topic: tractor-battery/voltage_timestamp
          payload: !lambda |-
            return to_string(id(sntp_time).now().timestamp);
  
# SNTP
time:
  - platform: sntp
    id: sntp_time
    timezone: "xxx"
    servers: "192.168.1.1"
    update_interval: 10s

Note: 10s is purely for testing and will be reduced once in production, and this works fine and I get this in mqtt explorer:

Untitled
Untitled240×105 3.8 KB

So now I want to consume the MQTT timestamp in home assistant and I think the right way to do that is via an mqtt sensor to read the value. So I have split out mqtt.yaml from the configuration.yaml and this seems fine (new to me though).

In that file I have the following:

-sensor
  - name: battery_volts_timestamp
    unique_id: e3ff039b-4c77-4bc0-8dac-66de50129002
    device_class: timestamp
    state_topic: battery/voltage_timestamp
    value_template: "{{ as_timestamp(value) | timestamp_local }}"

and nothing is appearing in the state for this entity.

What I am doing wrong? I am guessing its around the way the payload is referenced in the template, but not clear and this is the question I am focussed on.