Robbers used a JCB to smash open a safe before escaping with cash following a ram-raid last night.
Officers believe three men stole the key for the JCB on site before driving it through a wall of the offices, where they used the JCB to pick up a safe inside.
The robbers then used the JCB to to carry the safe over to another area of the site, where they smashed it against a wall to open it.
Disturbed by a security guard, they used the JCB to escape through the site's gates, before making off on foot.
A quantity of cash was stolen, including money raised by SITA staff for Macmillan Cancer Support.
No arrests have been made.
A spokesman for SITA UK said: "Fortunately, no one was hurt in this very serious incident, but there is significant damage to our premises and we have had to move our office-based staff to another site until the damage can be rectified.
"We are now doing what we can to help the police with their investigation.
"We feel very sorry for all the members of our staff who raised money for Macmillan Cancer Support, and for the charity itself, which has been robbed of this donation."
Anyone with information is asked to call police on 101 or call Crimestoppers anonymously on 0800 555 111