Merry Christmas! Problems with 3rd row seatbelts W124 estate

[billythetipdog]

The belts for the rear facing 3rd row of seats in my W124 estate have become difficult to use as the tensioner mechanism seems to click on immediately and will never let any slack out whether I try and pull hard or gently, feed it in then pull out etc. I've had a peep at the mechanism and everything looks ok; the coil springs are doing their job and retract the belts, problem seems to be with the tensioning mechanism not 'letting go', and I wonder if the ball bearing type switches that lock the tensioners may somehow be at fault

Has anyone any suggestions, thanks!