OK, we finally did it and the process was as follows (we have used Nick's advice. thanks!).
1. Freed the ends of the cable from the gear
and from the throttle.
2. removed the gas locker (stbd settee in the cockpit). To do that, we have removed the black rubber stripe that goes around the lid and then the screws that hold the locker in place hidden beneath the rubber stripe.
Then we had to free the gas tube by loosening and removing the large nut that holds the gas tube from inside of the locker. This is not the end though! The locker has a bottom drain for removing gas leaks
. This drain is connected to a hose that goes to a through-hull and has to be removed as well from the through-hull.
Access to the through-hull is possible through an inspection
panel on the port bulkhead of the stbd/aft cabin.
3. Removed the locker under the helm-seat on the port side of the cockpit (some boats have a fridge there). The locker is held in place by set of screws around its perimeter and, by a bolt at the visible on bottom of the locker. This bolt goes through the bottom of the locker and through a metal support frame that holds the weight of the locker. To gain access to the nut beneath the frame, we have opened the inspection
panel on the starboard bulkhead of the port/aft cabin.
4. We have opened the large inspection panel (held by Velcro tabs only) in the saloon
above the electrics cabinet, that allows access to all the systems beneath the helm-station. This is not a must as we have found that removal
of the two lockers as described above gives enough access to the damned cable.
4. We have pushed on the old cable from the above (through the port side locker) while pulling from the engine and gas lockers until the throttle end of the cable emerged in the opening on the port side.
5. Tied the end of the cables together (about 10 cm in parallel, tied by strong and thin twine and all of that covered by masking tape).
6. Pulled the old cable out and the new one in place, with help pushing/pulling through the openings available by the missing lockers.
7. We have found the old cable to be 33 feet (10m) in length. It may be of a different length on other L400s as we have replaced it with a cable of 30 feet (9 m) that was more than sufficient in length for the purpose).
8. I an considering to cut an additional access panel in the port bulkhead of the starboard cabin, near the stern, in the place near the gas locker. This will allow access to this area of the boat without the hassle of removal
of the gas locker.