Point 1: Subject to certain conditions, a creditor can't take legal action to enforce payment of a debt after a period of 6 years (not 12 months) has elapsed. However the debt isn't wiped out; the money is still owed and the creditor can still pursue the debtor for payment:
http://www.payplan.co...mitation-act-1980.php
Point 2: An employment tribunal would almost certainly rule that failing to return money (paid in error) to an employer constitutes 'gross misconduct', making it a valid ground for immediate dismissal.
Point 3: The employer can demand IMMEDIATE repayment of the money. The employer MIGHT agree to the amount being repaid over a period of time but is under no obligation to do so. (If full payment, upon demand, wasn't forthcoming, the employer could seek a county court judgement against the employee, which would effectively bar him from obtaining any credit).
Point 4: Debt is normally only a civil matter but criminal law is also relevant here.The Theft Act 1968 states: "Where a person gets property by another’s mistake, and is under an obligation to make restoration (in whole or in part) of the property or its proceeds or of the value thereof, then to the extent of that obligation the property or proceeds shall be regarded (as against him) as belonging to the person entitled to restoration, and an intention not to make restoration shall be regarded accordingly as an intention to deprive that person of the property or proceeds."
The Act also makes it clear that 'property' includes money and that the 'intention to deprive', referred to above, constitutes grounds for a charge of theft. The maximum penalty is 7 years imprisonment.
Chris