If you know this soldier was in the Lovat Scouts, then the likelehood is that this is the uniform of an officer of the Lovat Scouts, The Scottish Horse was originally formed in Natal South Africa from groups of ex pat Scots, The Scottish Horse wore the slouch hat with the added hackle of blackcock feathers, wereas the Lovat Scouts wore the slouch hat without the hackle and also wore the bonnet with dicing around, both the Scottish Horse and Lovat Scouts wore the three button cuff.
See some pics of Lovat Scout Officers in Inverness on eb*y just search Lovat Scouts.
I can't find any reference to Light Horse Inverness although a battery of RHA Inverness were attached to Australian Light Horse.
James