Autosleepers Dorset ES Seat Belts

Hi everyone. New to this forum (and to any other for that matter).
Anyone with a Dorset ES that can help with the rear nearside seat belt operation. (Unlike the Symbol ES, there is no hatch in the side of the bed box due to new regulations requiring a substantial bracket in the bed box fixed to the chasis).
I am old in the tooth but still find I need a minimum of three hands to hold the seat forward. lift out the fixed part of the belt. feed it OVER the slide for the seat runner. keep it flat and taut and carefully slide the seat back into the sitting posittion.
The belt sits in a very narrow gap between the seat bottom and the fixed slide.
There is a bracket fixed to the side of the bed box, (as in the Symbol ES) but is completely impossible to use as it is way below the belt where it leaves the bed box.
I have been assured by Auto-sleepers, (through the dealer) that there is not a problm with the belt or its fitting.
Any help please or loan of a few extra hands?