Thanks Neil and Merlin. It looks like that's him on the Great Britain in 1857, with presumably the letter "h" misread as "t". The person appointed to look after his business affairs in Edinburgh had died suddenly on 1st March 1857 so it looks like he might have grabbed the first boat that he could on hearing of this death. Incidentally he was a photographer and is known to have taken some early photographs of Melbourne.
As regards Alexander McGlashan on the Eagle in 1855 his age is given as 38 whereas Alexander McGlashon was born in 1811 and so would have been 43 at that time. However two of his children were John S (for Seton) and Margaret who were born in 1848 and 1846 respectively, corresponding to the names and ages of the children on the Eagle! As regards the 29 year old Jane McGlashan I do not know who she is (his wife was Mary Ann), although he did have a 10 year old daughter named Jane. Is this all just coincidence or is there more to this? My evidence that his wife and children remained in Edinburgh is an 1857 deed signed by Mary Ann McGlashon which states that she "with her children, who are all young were left in Edinburgh". Alex.