Buddy says: Where an amount in dispute cannot be quantified, the claim should be brought as a breach of contract claim, either in the county court or, if the individual is no longer employed, in the employment tribunal.
 
Additional information:  
The Employment Rights Act gives employees the right not to suffer unlawful deductions from wages and sets out a wide definition of what constitutes wages.  There must be a legal obligation to make the payment, whether the bonus is payable under the contract, by custom and practice or the result of an ad hoc decision.  Employment tribunals have the jurisdiction to hear breach of contract claims brought by an employee against an employer, therefore an employee who has suffered an unlawful deduction from their wages may bring a claim to an employment tribunal (section 23(1), ERA 1996.  However the claim must arise or be outstanding on termination.
 
In the recent case of Coors Brewers Ltd v Adcock, the Court of Appeal held in that, while it is possible to bring claims for unlawful deductions from wages for specific unpaid sums or for sums that can be identified, where the amount in dispute cannot be quantified, as was the case here, the claim falls outside the scope of the Employment Rights Act 1996.  Hence it was held that the claim should be brought as a breach of contract claim, either in the county court, high court, or, if the individual is no longer employed, in the tribunal.  Remember the Tribunal’s jurisdiction is limited to £25,000.