Finally got it working. I was basically trying to re-use the value of an id without ending up with two inputs for the editor in the admin page. Tried all the complicated ways and then discovered it was simpler than I had thought. …
I have a website page which has a region which allows the use to input details of a service.
The template is booking-form.html and in this there is a content input for a deposit for the service.
(Code, 1 line)