Can anyone advise whether my RF Switches are showing up in MQTT Explorer?

I think it is the entry under 433toMQTT.

You might want to check out this topic :

my automations.yaml :

- id: '1565365902541'
  alias: rfbridge_demultiplexer
  trigger:
  - platform: mqtt
    topic: home/OpenMQTTGateway/SRFBtoMQTT
  action:
  - data_template:
      payload: '{{trigger.payload_json.value}}'
    service: python_script.rfbridge_demux

In your case the topic would be : home/OpenMQTTGateway/433toMQTT