They are hydraulic
How did the cambelt slip, did it just jump a few teeth along the sprokets, or something else? The reason I ask is because a friend of mine with a Vectra 1.6 16v had a problem a few weeks ago where it was running very badly. One of many things I did was check the timing marks were lining up on it and they were. It turned out that the notch for the bottom pulley key in the crankshaft had actually worn/enlarged causing the pulley to slip around the crank, so despite the timing marks all lining up, the crank was sitting in the wrong position when they were all at their markers, i.e. timing way out. According to the local Opel garage, this was quite a common fault on the earlier 1.6 engines.
My friend had a lucky escape though, I got a local engineering shop to cut a new key into it, I put a new timing mark on the pulley, reset the bottom PAS belt pulley for the crank sensor and off he went.
A long boring story, but if you don't already, it might be worth while checking for wear in the notch in the crank when you do the cambelt just to make sure, seeing as Opel reckon it was a common fault.
i recon the when the new cam belt was put on the car the garage failed to change the tensioners which caused the belt to jump as you said. ill also let the garage know about the bottom pully key event though they will prob check it!
tell you my story,
was going along the duel carrage way at about 65 and i felt a slight judder so as you do slowed the car down and it seemed as if there was nothing wrong so i continued with my journey. the next thing i know i was about 100 yards from home when i felt a second judder and the car started to miss fire so i droped the clutch and let the car role home, the car seemed just to have slight miss but as i stopped the car had dropped to below 500 rpm which wasnt good!