While it is usually unhelpful to discuss the wrong alerting rules, in this case it does not matter: an unalerted double is lead-directing in both England and Wales, and in North America, so if it means something else then it requires an alert in both jurisdictions.
[Note: the original poster said he was from the UK. Scottish and Irish alerting regs are different. Since he did not specify I am assuming England or Wales.]
I think that Henry's post is somewhat unsympathetic to the side who might have been damaged. As South I am sure at the table that it sounds as though North is void in diamonds, and to proceed to the five-level with his load of junk, most of which seems to be waste paper, would be very optimistic.
Furthermore, his suggested redouble is normally played as first round control, so KJxx does not seem the right holding to redouble! Given the apparent lead-directing double of 5
I believe a lot of pairs would now go wrong.
Perhaps a better approach, rather than criticising N/S's bidding, is to consider first whether there is MI and second whether there is damage.
If the double was lead-directing then it does not require an alert. It is difficult from here to know whether it was lead-directing. True, the player does not seem to have had the correct hand for the call, but that proves nothing.
The TD had to decide whether East-West had an agreement that the double was something other than lead-directing. To be honest the evidence given here suggests to me that they did not have such an agreement. If that is so then there was no MI, and thus no redress would be offered.
Let us suppose that they did have such an agreement, perhaps that a double of 4
shows the other two suits or something. Now South's hand improves dramatically, and I would adjust, perhaps to
.. 15% of 7
+ 70% of 6
+ 15% of 4