I think it is totally pointless having full length curtains over any heater - the heat just will not get into the room.
If the heaters are functioning correctly, the curtains will not catch fire, but the thermostat will cut out, stopping the heaters working.
I found this:
You should never cover the surfaces of the heater or obstruct the air outlet grilles as this will cause temperatures of the product to rise and the safety cut-outs to operate.
So you need to shorten those curtains!
This site provides useful information and a telephone number:
http://www.dimplex.co.uk/consumer/faq-storage. htm