Janet has just sent a photo of her newly-installed blinds. She installed them because she was concerned about heat rising behind the curtains. This way, even with the curtains open, a well-fitted blackout blind or thermal blind will reduce heat loss.
Alternatively, if the situation allows, why not double up on window coverings by combining curtains, blinds or shutters?
With a suitable window ledge or a small shelf below the curtains, heat from the back of the radiator goes into the room rather than up behind the curtains. This is even more effective if you put radiator panels behind the radiator to reduce loss of heat into the external wall.
For more ideas, have a look at the HOBBS report on this blog (2nd tab). You can download it for free, allowing you to click on the contents page to read one of the case studies or any other part of the report.