It appears from your photos that the stock is bent immediately above - or just at - the upper bearing where it exits into the steering
flat. How could that possibly have happened?
As JimBunyard said the boat
has a full skeg, as your attached drawing shows. Stan Huntingford's designs were sound, and he would have designed the rudder
arrangements well, and simple enough that Cooper
Yachts could build them easily. That your rudder
turns, even if it feels "resistive", would indicate that from the upper bearing right down to the gudgeon at the bottom of the skeg, things are alright. However, the friction that makes the steering
"resistive" will be found where the stock goes through the upper bearing.
So it looks to me as if you are faced with a sorrowful situation:
The top bearing should be replaced, and a new straight stock should be provided. That cannot be done without withdrawing the entire rudder, c/w stock, FROM THE BOTTOM! That will require hauling to gain access to the gudgeon fastenings.
The bent part of the stock will not, obviously, go through the steering well and its bearings as it is. The stock must therefore be cut just above the upper bearing. The collar should come off to do this. Even when so cut it is likely that the cut end of the stock will damage the bearing at the bottom of the hull
as you withdraw it.
Cutting with a torch would be extremely dicey working within the steering flat so a saw is preferable, but, as I'm sure you know, sawing through stainless steel
- which is what the stock will be - has its own problems!
Even when the rudder and stock are out of the boat
it is unlikely that welding a new piece of stock to the part still in the rudder blade will be possible because proper axial alignment will be a problem. It would be, if it is possible at all, a job for skilled professionals.
You ask specifically for a cause. This damage has originated from WITHIN the steering flat, I'm pretty sure. It has nothing to do with hitting a reef or any other such thing.
You ask specifically for a cost estimate. IMO - without doing actual costing - I would suggest something in the region for $40,000. If you do not already own this boat I would suggest you find another candidate for your ownership
All the best to you.