Although I removed the seat from the car to do this in hindsight this is perfectly doable in situ.
Cover is tucked into a groove round the outside of the seatback and is removed by rolling it towards the front, the harness slot untucks in a similar way. The bottom of the cover looked like it was glued in on mine so I left that where it was and just pulled the cover and foam away to give me access...


Removal of some screws and the black cover comes off to reveal the mech.....


Allowing me to remove the broken bits......


And perform some surgery.......






And refitted.......


Some reassembly and refitting later we now have a  pair of working releases. Because of where the break is only the top 2 screws are visible and then only when the release is fully pulled up.
 Hope this can be of help to others.

Well done  You managed to fix the problem without having to go to the dealer, I bet it is now stronger than original.

Just checked back in and discovered I'd got 4 unanswered dms asking how to 'de-skin' the Sabelts to get to the release handle 
So hopefully this picture and a crude drawing can help those who've asked.
Notes:
The groove runs around the outer edge of the seat including the harness slot.
To refit roll the edge back into the groove