UPDATE 1-Scottish refinery fire extinguished

(Adds level of production at time of fire, spokesman from Central Scotland Fire and Rescue Service on unit affected)

LONDON, May 10 (Reuters) - Scotland's Grangemouth refinery, which closed due to a two-day strike last month, was forced to delay its return to full production after a fire broke out on Saturday, the plant's operator and the fire service said.

A spokesman for the refinery's operator Ineos said the site was operating at roughly 80 percent of capacity when the fire broke out in the diesel line.

"It is expected to extend the timing of the restart at the site," Ineos communications manager Richard Longden said by telephone on Saturday.

The 200,000 barrels per day refinery was shut down late last month because of a two-day strike over proposed changes to pension arrangements.

Ineos was aiming for full production in the coming weeks and was working out a schedule of talks with the union. Longden declined to specify whether the unit affected by the fire was still operating.

A spokesman for the Central Scotland Fire and Rescue Service said the fire broke out when some oil spilt on to pipework.

"As far as we are aware it was the hydrocracker," the spokesman said, referring to a unit that produces diesel and jet fuel.

The fire was quickly extinguished by the refinery's emergency team. An investigation is being carried out to determine the precise cause.