It turned out to be the NTE again (lightning maybe?), but this time the engineer replaced the filter as well. Didn't think much of it until after he'd left.
Checked my stats, and I've lost 6MB/s in downstream sync speed.
He's fitted a MK3 filter, and left with my MK2 filter (that I purchased!).
Is this the likely cause of the sync speed dropping? Or maybe a dodgy NTE replacement?
ZeN Unlimited Fibre 2