I've just seen this thread and thought I'd add my 2c worth in regards to reloading a Mk3. No1 as I've got 1 with a heavy barrel.
I've set my OAL at the same length as the original military round and use 180gn Sierra Pro Hunter pill with 45.5 gns of AR2209 and this produces around a 2" group at 100yds using the battle sights over a rest.
I've also got a hold of 180gn Highland pills that I'm going to try as well,regards