So, this song is clearly aobut the "Alexander Kielland" tragedy. The Kielland was a "floatel" (floating hotel) used by oil workers off the coast of Norway. 123 men died when a storm struck the flotel, causing one of the legs to break off and the remainder of the vessel to capsize.

I was able to find a list of the deceased from the accident (click), but Billy Johnston (or any variation thereof) is not a name in the list.

British deaths included:
- Alan Beggs (27)
- George Terence Collins (37)
- Brian Owen Dowson (36)
- David Lawrence Elliot (35)
- Michael Fleming (37)
- Brian Graham (31)
- John Michael Harris (29)
- Edward Laxon (52)
- John James McGrady (43)
- Richard J. D. Milne (40)
- Robert A. D. Morrison (25)
- John Christopher Murray (33)
- Patrick Joseph Pender (42)
- John Richard Phillips (30)
- Barry Bruce Pickup (36)
- James Edward Poulter (39)
- Thomas Prior (36)
- George Christopher Purcell (31)
- John Thomas Richardson (40)
- John Joseph Tegowski (29)
- Paul Ian Thomas (30)

None of those names sound much like Billy Johnston/Johnson to me. Additionally, very few of the deceased would have been old enough to have fought in Korea, as told in the song. Fighting in Korea means they would have been ~20 in 1950-1954. That age means anyone who would have perished in the disaster would have been about 50 at the time of the accident. Only one name on that list is 50.

That leaves me to think that Billy Johnston is a fictional name made to fit with the tragedy of the song at the time.

I am, of course, always willing to help with lyric transcription, etc, but I was unable to find any songs matching any names I thought of online.