Impossible to answer. just for a start nobody knows if you are trying to heat a one bedroom terrace house with decent insulation or a 20 bedroom mansion with no insulation.
The heating and hot water switched off but the pump remained running. I know this should not happen but I am trying to find out how much it has cost me when it did happen.
The house has 5 bedrooms and is detached but that would not seem to make a difference given that only the pump is running.
Look on the pump and see if it has a wattage value on it, if it has then you can work out the cost from that. (it would also help to know what you pay per unit if you want an accurate guesstimate)
assuming the pump is drawing about 100wats and your cost per unit is about 12p then 16 hours would cost 19.2pence
Though as you can see the values it's worked on are assumptions. to get an accurate figure the pumps rating and the cost per unit of electricity is required.
monk certain boilers like combis have pump overrun to dissipate the heat in the system and save wear /tear on the heat exchanger.untill the water has cooled down the pump will keep circulating the water