i worked in hr for a while (sorry! but it was running an engineering apprenticeship scheme..)
unless you can find something specifically related to over or under payment in your contract (which you probably wont) YOU are liable to identify the error and repay in full to your employer at their discretion - this is the same as banks if they accidentally transfer money into your account, they can ask for the whole lot straight away so no good if you have spent it.
if you are on a regular salary that doesnt change month on month, technically you could be accused of fraud as you "must" have been aware of the incorrect amount - i am not saying you were but that is the potential employers stance!
none of this sounds great i know BUT this situation occurs more than anyone realises. if this has occurred over many months (and to others - see if you can find out) there is obviously an inherent process problem with your employers payroll system - this is what you should concentrate on not "poor me, i spent all the extra money you gave me and i cant pay you back" PLEASE NOTE I AM NOT SUGGESTING YOU ARE SAYING THIS, JUST AN EXAMPLE. focus instead on having a structered discussion on wanting to help the employer identify where the problem may lie so that it doesnt affect you or your colleagues in the future - face to face if possible, try to explore whether a computer problem or data input error etc if you can build a rapport in this way you are more likely to get a friend willing to bend the system to reduce your re-payments, in some instances if you go in too heavy on the rules/unions etc the employer will use the formal rules against you and in this case they have your money before they pay you so can take out whatever they want - try the softly softly approach first, even if it means apologising for what isnt even your fault! i once knew an hr manager who was the nicest and most supportive guy in the world to all staff until they tried to threaten action - no mercy for a few that wrongly crossed them
