Finally got to the top of the *things to do* list:-)
With the door fully closed, taking out the main drive roller is simple. The slats are linked to the roller by 4 curved aluminium connectors, each secured by a pivot pin. The pins are held by two nylon rings which form a surface for the slats to *roll up on*. The rings are retained by large pop rivets fitted in a longitudinal slot in the drive roller.
I found it was possible to slide the rivets along to gain clearance and separate the pins from the nylon rings which released the slats. Either end of the drive roll is an octagonal flange slotted into a welded cup and retained by 3 grub screws.
With the screws removed and the motor disconnected the drive roll can be lifted out of the cups and manoeuvred clear of the support brackets for examination. Best done by 2 people.
The tubular motor is supported at the supply end by a plastic moulding and at the drive end by an aluminium casting which engages with internal flanges in the drive roll. 2 pop rivets are used to fix the casting to the roll. Once the rivets are drilled out, the motor and casting can be withdrawn.
The motor drive is by a plastic spline secured inside the ally casting with a grub screw and a retaining plate.
In my case, the epicyclic gearbox was smashed thus removing both mechanical and electrical means of opening the door!