I've been having the same problem! Looking at your picture, if you put your finger where the orange bit is and lift the blue plastic catch upwards and then pull it'll come apart. I have just spent a few hours trying to separate this plug. Hope that helps..
You maybe able to answer my question.
I've swapped the double passenger seat for a single drivers seat. I notice this blue plug goes to the centre seat seatbelt reel (on the drivers side it goes the the seatbelt anchor point)
So now I don't have a centre seatbelt reel the plug is redundant although I've connected it to the seatbelt anchor on the passenger side (was a drivers seat). I now have a fault on the dash 'seatbelt/airbag fault'. I assume its looking for the centre seatbelt reel.
Would you know how I would over come this. (I notice on the double passenger seat the 2 seatbelt anchors are not sensed, just the reels)
Just spoken to a local taxi conversion company who also suggested putting the seatbelt under the seat as the best option. He mentioned resisters but said they often fail.p.s. I'm going to ring Allied vehicles in the wk. As our pug expert originally came from them. (they do disabled conversions, involves removing seat etc..)