Is there any rough estimate when the Inverness portion will become double headed 73s rather than the hired in 67?
Likely when they have enough + spares.
There are 4 in Scotland which would be enough for it, but that leaves no spares to cover failures/exams. I suspect they'll hold off until 970 and 967 return from Brush so they have that cover. When that'll be is anyones guess at the moment.
There's also now the 0Z73 path in tomorrow from Craigentinny to Doncaster Down Decoy GBRf (a typical staging post for moves to Brush) which suggests one (or more) of the four currently in Scotland is heading to Loughborough for a fix.
Based on reliable info elsewhere, 970 (Dellner mods) is probably another week or so away from release, and 967 which went in last week wasn't expected to be a big job I don't think. There will then be whatever 73(s) are heading South tomorrow in too.
So I reckon it's going to be at least mid-Feb, if not end of Feb until the 73/9s are at full-strength (or near enough) back in Scotland. The other thing to factor in is one 73/9 is likely to be allocated for extended periods to test the Mk5s (including mileage accumulation runs) - so they're really going to need all six back up there, and probably 47727 as a 7th diesel option before they can even consider using 2x73/9s on Inverness on a regular basis and hand back the DBC 67.