I would think Ladybirder’s estimate to be on the low side.
Assuming that the water is flowing out of the tap at 3 litres/min, over 24 hours this will represent a volume of 4.32 cubic metres – at around £1.50 per cubic metre works out at around £6.50.
So Ladybirder’s estimate would be correct based on around 1.5 litres/min flow.
Of course depending on the actual flow, the cost could be more or less than this amount.
A kitchen tap should have a flow rate of about five litres per minute or 300 litres per hour. Leaving it on for 24 hours would thus see a discharge of around 7,200 litres. My water company charges £2.12 per cubic metre (which is 1,000 litres) so the cost of your error would be a little over £15.
If you live in an unmetered property of course you will pay no more than your annual bill.
"i inadvertently left a tap running..." gives me the impression it was left fairly full on rather than dripping or dribbling. Perhaps the OP could help.
Our water company also puts a charge on for waste water like toilets in with the water charge, although you may pay say £3 for water there is also £5 on top for waste.
Our bill comes in water used and waste, and the waste is higher than the used part. They have to charge for sewerage and that is in with the amount of water that you use.
Thanks guys, I feel a lot better now. I was anticipating something awful like £50! Yes, tap was outside. The other end has a controllable on/off control but the water pressure had forced this off so water flowed out all night.