Well, this is interesting. Last night was the 2nd overnight since I replaced the ECI modem with the unlocked HG612 and we had a resync / reset.
I can't cut and paste the stats because DSLStats is now running on a raspberry pi and I don't have any spare ports on my router for the line2 connection so it's a private IP only, BUT...
G.INP has been re-applied. My Interleaving has gone down from 1100+ to 16 and bearer 1 has appeared. In addition to this the speeds have now resync'd at 20Mb up, 74Mb down. Not quite back to the 80/20 that it was before G.INP was applied but we're not far off. I do wonder if I may see further resets in the coming days that might gradually get it back up again? Who knows.
In addition, I'm thinking that the firmware may have been updated on the router. I need to go and check. I'm sure I had killed the BTAgent (need to telnet in and check that too) but my Zen portal is telling me exact sync speeds like 73919 and the modem stats are rounding them up to 74000. That seems new to me.
Will double check that last bit.