I am just looking at bsac reports.
2015 1 solo out of 9 fatalities
2016 1 solo diver out of 11 fatalities
2017 2 solo diver out of 10 total fatalities
As noted by Ucarkus, total numbers are meaningless if we do not know the percentages of the total populations.Sample is all divers, given small amount of solo divers, they should not be presented at all, I would expect 0 incident per year, yet this is not the case.
A few years ago, someone remarked that over the last couple years, there were just as many cave diving fatalities with certified cave divers as non-certified divers, and he concluded that it showed there was no benefit to cave training. In reality, since probably 99% of cave dives are now done by people with some level of cave certification, the fact that the fatality numbers were about the same showed that a remarkably high percentage of non-certified divers die and a remarkably low percentage of certified divers die.
Since the overwhelming majority of divers dive either with buddies or in groups, if all other factors were equal, you would expect the overwhelming majority of the fatalities to be on dives with buddies or groups. I have no way of interpreting the relative danger of solo diving from those numbers.