Hiya,
Up until this morning I'd been using the B028SP10 firmware (I'd tried both the original and unlocked variants) with no success.
I realised this morning that I had not tried the B030SP06 version, so tried initially flashing the locked version and the modem synchronised straight away. I then tried the unlocked version and again, the modem syncronised as quickly as the ECI modem openreach supplied me.
To be sure I then re-flashed B028SP10 on the HG612 and sure enough it failed to synchronise. With B030SP06 back on, it synchronised straight away.
I assume the problem might be something to do with my telephone line or maybe the equipment in my cabinet, but either way the B030SP06 firmware seems to be working.
Thanks again for the feedback!
Cheers,
Chris