Yes. You should receive at least 5.6 weeks' paid holiday a year.
In the past, some agencies tried to get round this by saying that your hourly pay rate included holiday pay and, therefore, that they did not have to give extra pay if you took leave. However, as a result of a decision by the European Court of Justice (ECJ), this practice (known as 'rolled-up pay') has been outlawed, and agency workers have a right to receive payment on days they take as holiday.
As an agency worker, your hours and pay may vary considerably over time. If this is the case, your earnings over the most recent 12-week period are divided by the hours worked over the same 12 weeks to give you an average hourly rate and determine your holiday pay. If you did not do any work at all in one or more weeks, you simply discount that week and move to the week immediately before it, until you have a total of 12 weeks' work and pay on which to work out your average hourly rate.
(Note that the government plans to change the law. From April 2020, holiday pay for workers whose pay varies will be averaged over 52-weeks instead of 12.)
In addition to their statutory holiday rights under the Working Time Directive, agency workers have extra rights under the Agency Workers Regulations. In particular, since 1 October 2011, temporary agency workers have had the right to the same holiday entitlement (pro rata to their hours) as a comparable direct hire employed by the hirer to do the same job. This right applies once the agency worker has completed a 12-week qualifying period.
For example, if the hirer's employees doing the same job as you are entitled to 35 days' holiday a year, you will also be entitled to 35 days, once you have completed the 12-week qualifying period.
If you work part-time, your leave entitlement can be calculated on a pro rata basis. In other words, your entitlement will be proportionate to the hours you work. For example, if you only work six months rather than a complete year, you will be entitled to half the annual entitlement.
After qualifying for equal treatment, you should be treated the same as the hirer's employees when requesting and being permitted to take holidays. You will also be entitled to equal treatment on enhanced pay for working on bank holidays or public holidays, and the right to time off on bank holidays or public holidays.
Your employer is not allowed to “buy off” your rights under the Agency Worker Regulations. For example, it is against the law for your employer to give you less holiday than your directly employed counterparts but compensate you with a premium hourly rate.
If you are in doubt about your entitlement, seek further assistance from your union, or an advice agency if you are not a union member.