Hi,
I would consider time spent as an agency worker lenght of service. Although not an employee during this time, as soon as you have 12 months service in one company, the employer is responsible under the Unfair Dismissals Act.
Recent case law has shown that agency workers can be deemed an employee. Some info on these cases can be found here: [broken link removed]
If you brought a case on it, issues such as control (were your duties determined by the employer and did they have alot of control over your work the way you woked, the tools you used etc) and Integration would be considered in a case (were you treated the same as regular employees e.g. invited to Christmas parties, offered other benefits accociated with regular employees) would be considered.