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Brigade Games in the USA now produce that kit. It is available from them. For a while you could still get it from HLBSC by asking them nicely (I know of someone who got one from them long after they stopped listing it), but now that Brigade have the rights to it that source may have dried up.
I had a look on their site but when I did 'submarine search' all I got were a couple of ACW subs.Do you know what it's called?