According to the Commonwealth War Graves Commission site that Phil pointed me to Walter Scott's date of death is listed as the 27th of April, 1941. This info I previously didn't know. Using a search tip that Phil suggested, I started searching for ship names that could have been mispronounced or misspelled, and I homed in on ships that were sunk on that date.

It turns out that Wikipedia has a listing of probably every ship that went down, any time, for any reason - [en.wikipedia.org]

(One day I should find out how to make links appear properly)

Anyway, the list of 27th April 1941 included the SS Slamat. She was a Dutch liner but was operating as a troop ship in the Mediterranean Sea, and was part of the Allied evacuation from Greece. The full story can be seen here - [en.wikipedia.org] The action was particulary horrific because after she was bombed two of the rescue vessels were also sunk with survivors from the Slamat on board. In the end almost a thousand people lost their lives. Of the 720 soldiers and crew on the Slamat only 19 survived.

At a guess you could see how the names "Sloop Slanto" and "S.S.Slamat" could be transposed. But there are a couple of other points. Walter Scott was declared "missing" and remained that way for quite some time, much to the distress of his family. This would tie in with the mayhem of that event. The other aspect, and I'm not going "conspiracy theory" here, but what if the disaster of the loss of the Slamat was too much for the authorities to release to the public at that time so they buried it for a bit. It wouldn't be the first or last time that happened - the war was going particularly badly for the Allies at that point, so I wouldn't blame them for trying to temper the flow of bad news.

Anyway the upshot is that there is no record of any boat called "Slanto", and the evidence is currently pointing at the "S.S. Slamat" as the ship that my grandfather died on. Ah well, more research to be done!