Based on 15p/kWh (which is likely to be somewhere near what you are paying), running continuously (at full power) for 1 hour will cost �1.80. Multiply that by 6 hours a day and then by 7 days a week, works out at a total of �75.60. With 9 rads in a 4 bedroom house, I would suspect that the heater is working hard to heat the house (is on more than 80% of the time). Some of these boilers have indication (light on) when they are heating.
Add to the above figure, your other electricity usage (cooking, baths, lighting etc) and you are probably look at a bill not far short of �100 a week.
Electric heating is expensive, as you have discovered � my understanding is that these heaters are normally used as a back-up for gas heating, where guaranteed hot water is required.
The price for gas heating is around 4p/kWh. I would strongly recommend you considered adding gas heating, leaving your electric heaters for emergency only.