I fully agree with Lutz on this one,
Alko have been manufacturing the hitch and its locking mechanism for many years, and thus its size and operation is not only specified by Alko, it well known through experience.
The caravan manufacturer is responsible for ensuring that their additions to the chassis do not impede the operation of the hitch, and the associated braking systems.
In my view there should be clear warnings on the hitch or on the caravan or both about the use of the lock whilst travelling.
Looks like you guys may be right. I made a claim against Alko and received quickly this reply.
"I am sorry to hear of the issue that you have experienced but would assure you that there is no design fault with either the lock or the stabiliser.
The problem has arisen I would surmise - having not inspected the unit - because the A frame cover has been fitted too far forward. The A frame cover is fitted using slotted brackets so that it can be adjusted and if it is fitted too far forward then when braking occurs contact can be made by either the stabiliser./coupling or indeed the lock.
As this issue is not due to any manufacturing fault or material fault with either of our products then I am afraid there is nothing that I can do in respect of any claim."
I haven't checked the location of the A frame cover yet, but it looks like it's over to a warrenty claim next.