I've now flashed my second HG612 with the B030SP06_unlocked firmware, and it's looking good. I'm using it now (on TT ADSL2+), and the connection speed when it started up was the highest I've had for many months. Everything else appears to be working normally, and DSLstats is plotting all its data as it should.
I feel slightly uncomfortable without a GUI interface, but that aside, this looks like the best firmware yet, and a first class job by HW, for which, many thanks.