Water company bosses may fit back-up alarms to sewage pumping stations after mice caused a pollution scare.

Rodents had chewed through a cable to an alarm which should have sounded after pumps failed, leading to raw sewage overflowing from an unmanned pumping station.

At least one fish and other wildlife died when ammonia and other pollutants rose by up to seven times normal levels over a 200-metre stretch of a stream at Stakers Lane, near Horsham.

Southern Water was fined £15,000 after admitting polluting the River Adur following the incident in May last year.

The company appealed against the fine at Hove Crown Court yesterday and had the amount reduced to £10,000.

Richard Banwell, for the Environment Agency, said the stream had suffered a "serious acute pollution event".

He said at least one perch had been found dead in the river three days after the incident and 18 other categories of river life had been affected.

Clifford Darton, for Southern Water, said deep wet wells at the pumping station were designed to cope with any overflow if pumps failed. They were fitted with an alarm that would alert the company automatically and give staff two hours to act before the wet wells overflowed.

The mice or rats responsible had shown "great dexterity" in reaching the alarm cable, Mr Darton said.

It was rare for pumps to fail and the rodent attack was something which could not reasonably have been foreseen.

He added the company had replaced the wiring with armoured cabling and had re-routed it to make it less accessible to mice and rats.

Mr Darton said: "The facts do not merit a fine that was almost at the top of the magistrates sentencing powers of £20,000."

Judge Austin Issard-Davies, upholding the appeal, said: "Taking into account the plea of guilty, we find the magistrates have sentenced at the upper limit of their powers.

"We are concerned the alarm telemetry is not tested more regularly and there is not a secondary alarm as a backup."

After the hearing a Southern Water spokeswoman said the company would take the judge's comments into account when considering what action to take to prevent similar incidents in future.