Just slipping in first gear?
It would be more usual for it to slip in a higher gear, third and fourth are most common due to the way the gear ratios and the torque interact on the clutch plates.
Have Vauxhall had a look, or better still try to find a van rental company that also have a workshop, they are more likely to take it for a quick drive to let you know, and will probably know of the common problems without giving you dealer bull.
Good Luck