I get that correctly: 1. Zambia has benefited from more than £3 billion of debt cancellation and 2. It has to pay £28 million for the £15 million it borrowed from Romania in 1979 due to a loophole in the international agreements ? That's 2.3 % interest p.a., right?

PS: You are off-topic. 

My point isn't that Zambia "benefited" or not (where did the money go? but THIS is another "off-topic" story).
My point is : How Debt Advisory International by "revamping" a cheap debt bought for $4 millions up to $40 millions does anything "good for the economy" or bring a "revival of dynamism" or enhance the "values and attitudes" which enable the economy to thrive?
This is just plundering the dejected, the fact that many african countries are hopelessly incompetent economically is no excuse for piracy.
The causes are beyond simplistic economic rules, they won't learn by being punished, especially since the ones individuals punished are not the ones who squandered the ressources and benefited from the loans.