You should be fine with that Stu - I'm not at all surprised it has been replaced - a LOT of cars were affected.
Only thing to be be mindful of is that initially BMW did not know the cause of the problem and replaced Nicasil blocks with the same. It was only later they discovered that the high sulphur content of the petrol at the time was turning into sulphuric acid during the combustion process and literally eating away at the cylinder liners.
On some cars where the problem was discovered early they have had 2 replacement blocks - initially a Nicasil and then the later improved material liner that was more resistant to attack.
In conjunction with this, the petrol companies have helped by refining out more of the sulphur ( low sulphur fuel ).

Sounds like your leakdown test has shown the engine is fit anyway Stu so
you can get on with the important job of driving it

The early 1.7 Yamaha engines used in the Puma also suffered from a similar ailment as a girl I work with discovered to her unfortunate cost

Cheers