Right get a really small flatblade screw driver and put it in where the two plastic covers meet and twist it to prize them apart be gentle tho as they are flimsy (can always pick another cover up from scrappies if you do break it). Once you have the cover off you should see the white locking clip which should be tensioned by a small spring, what happens is the spring comes off the little nipple and the latch doesnt lock in to the belt if you reseat the spring it should lock in again. Then just clip your covers back on. Worked for me anyway mine did the exact same thing.
I've just had to do this repair on my car. I tried to carefully prise the old socket casing apart, but it ended up too trashed to reuse. I found the little spring inside had indeed jumped it's seating but sadly was too bent up so in the end the whole thing was u/s.
I checked and the replacement socket and pretensioner units are available from Vauxhall but these are £££s. I could only find passenger ones on fleabay but Autoparts24 in Germany had one for £60 which got me sorted.
Vauxhall updated the socket - the original part number 90359922 (drivers side) was replaced with 90442384.
I'd noticed that Vauxhall Green had the passenger side listed (90442383). Turns out they also have some of the repair kits too (9199753). They are a much better design but you need a bit of skill to fit them as the old socket pretensioner cable lug needs to be drilled out. A shear-bolt is used in the replacement. I'd say it's still best to remove the seat from the car to do the job properly. Care has to be taken with the pretensioner. You must make sure you fit that safety locking clip first!