Today I had another resync and another drop in upstream sync, it' now dropped to 4387kbps with 46301kbs downstream.
So I dusted off the HG612 and tried that on there, that gave 41415/6101kbps, so it seems around 4886 loss on the downstream, but a gain of 1714 on the upstream.
It's not causing me a problem at the moment but if it keeps on dropping then I'll have no upstream left, at least I'll be able to raise a fault.
I've added some stats, two for today, one from the HG612 & Zyxel, and one from HG612 in 2013 when I had an upstream of 11295 kbps