Elsewhere on the same site:
http://www.noboheatinguk.com/pdf/s14_e4eu.pdf
Your model has variable output from 0.5kW to 2kW, depending on how high you turn up the heat. So anything from about 5p to 20p per heater per hour.
20p per hour is very expensive, especially for multiple heaters. Hopefully you won't need to run them all at high heat very often!