Well, they aren't storage heaters, so you can ignore most of the responses in this thread.
They are not so different from bar heaters, they have an element inside that heats up when switched on.
To calculate your running costs, take your leccy price - 10.49 pence for one Kilowatt/Hour - and multiply by the power output of the heater. This should be on a rating plate somewhere on the heater. If you can't find it, try to look up your exact model online. Is it one of the ones you linked to?
If you have 0.5Kw heaters, that's 10.49 x 0.5 - 5.245 pence per heater per hour.