Thanks Kpt. Lehmann, I had a few CTD's (which maybe something else) & a small ship that refused to sink no matter what I threw at it, went back hours later still there half submerged!
Ok just a quick update again on this problem hoping that someone may know what this is:
As I was saying once locked, the ship then appears on the map, what I have since noticed is that if I lower the scope all ships are replaced by the hydraphone lines, which is normal, except for the Fiji, it's still showing the ship icon though it
very slowly fades out but is
not replaced with the hydraphone lines, when raising the scope all ships appear again except the damn Fiji, unless ofcourse, I lock on to it!!

This is most frustrating, hopefully some kind soul will say "
I know what that is!" .. well, here's hoping!!