Hey Neil,
At this stage the last updated column isn’t currently supported in our automations, as you have come to realise.
I encourage you to add your vote to the existing feature request here → Use Last Updated column in date automations - #3 by AllisonShores 
To work around the limitation in an feasible way, I am interested if you could instead take advantage of our board views feature or conditional colouring capabilities?
Option 1:
Create a filtered board view for items that are RED and fall into the current month (or next month):
Option 2:
Similar approach but just using conditional colouring to identify those items with the same filtering type:
I understand it isn’t exactly what you’re looking for but I’d love to hear your thoughts!