Its documented that multi-class characters gain HLAs at a higher level, but there is no specification as to when. Perhaps it would be good to clarify this, so that players can decide the validity of multi-class characters

Mutli-class characters start gaining HLAs approximately after gaining 5,100,000 xp. It may slightly differ a bit for different classes.

Single class characters start to gain HLAs approximately after gaining 3,400,000 xp. Again, it may be slightly different for different classes.

Just to clarify, is this both 2 class multis (eg. F/M) and 3 class multis (eg. F/M/C), or just the former?

It's generally true for all multi-class characters (including 2 class multis and 3 class multis). They require 50% more xp than single-class characters to start gaining HLAs.

OK, thanks