Grand C4 Picasso Seatbelt tensioning -- what should it feel like ?

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I'm going to wind up returning my new toy to the dealer for rectification of the wonky electrics under warranty, but before I do, I am trying to establish if the seatbelt pretensioning is working correctly, oor not. Never had a car with this feature, so don't know what to expect. Once underway, the belt is fairly feebly reeled in and then judders a couple of times, as though there were some duff ratchet teeth within the mechanism. Occasionally it will then repeat the process while motoring along. The passenger belt sometimes does; sometimes doesn't. On a couple of cars I test drove, the belt seemed to pull in much more firmly.

Which is correct ?

Mike.