FTTC low upstream

VDSL2-17a connection
Recent new line install with fresh master socket and faceplate. In good circumstances, the line can sync at

18.828Mbit/s downstream with SNR 6.5dB
1.188Mbit/s upstream with SNR 6.40dB

I'm quite puzzled by the low upstream, router will sync as low as 600kbit/s. Line length is long and predictions on BT wholesale checker are way off on the upstream. Underground cabling was installed in the 70s.
Would appreciate any thoughts (other than move house!! haha) ?

Would need a bin plot to get a good handle on what is happening to the frequencies used, it may be that one of the upstream blocks is being heavily impacted due to line characteristics or a noise source.

If you have powerline plugs I'd recommend unplugging them and seeing if the sync improves.

Thanks Andrew.

There are no powerline adaptors used in the house or any internal phone wiring. The line has been tested by an engineer on the master socket just this morning. It's an odd one... I highly suspect the cabling to the cabinet isn't great quality. Due to the low upstream, we'll probably just look to move back to ADSL2 at the end of our contract. We're with Plusnet and they've been fantastic at handling the fault/query..