There is no statutory requirement to pay relocation allowances, but it is quite common for employers to have a contractual policy on such allowances – this is especially likely in unionised workplaces.
If you have not been paid the correct amount, raise the issue with your manager. If you are a member of a trade union, consult your union representative. If the matter can't be resolved by these informal means, you may have to raise a formal grievance using your organisation's formal grievance procedure.
Where a policy or formal agreement on relocation allowances has been incorporated into your individual contract of employment, you will be able to pursue a legal claim. Your claim, which must be brought promptly and within the relevant time limits, will likely be for breach of contract and/or for unlawful deduction from wages.
Where a group of workers are affected, it is usually best to begin by organising yourselves to try to tackle the issue with your employer collectively, via your union if one is recognised where you work.