Peter
In my experience Clients have used “distraction” techniques to make sure the tenants are not just dismissing the letters because they are from the Council and obviously this suggests they are about the rent arrears:
1) Use a different type of envelope, different colour, no window, different size, different shape etc
2) Use a postcard with a picture of a gas explosion aftermath on the front and a simple short message on the back
3) Mark the envelope with a banner stating “This concerns your safety”, “Urgent Gas Safety”, “Gas Safety Information” or such like to grab the attention
Hope this helps
Trevor