Agreed, odd is the word I'd use too, even with my limited understanding. If anything, the downstream has a little more SNRM headroom than it would normally have. I'm sure it sits at around 6dB typically but now it's at around 7 - 7.1dB, increasing the attainable rate.
The upstream output power has taken a nose dive since I re-synced after the test last night. It's now at -7.2dBm.
I've just spoken to IDNet again, still not convinced by their customer service, and ran through much the same as I did last night. So hopefully an engineer visit will be arranged this time.
Service: FTTC Unlimited (55/10)
Modem/ router: Billion BiPAC 8800AXL
Current sync: 55000/ 9999