so just an update in case anyone else gets similar problems
baby_frogmella was right on target with the congestion thing. After a couple of to and fro's with Openreach, the line got moved to another VLAN and that's improved matters greatly (I was seeing speeds as slow as 30Mbps in the evenings before the move and now I'm pretty much always over 200Mbps)
Apparently the complex faults team are still looking at it as there's some undefined problem on back-end systems, but it's now largely fixed
As an aside, one technique I found useful for gathering statistics to demonstrate the problem was using iperf3 (https://iperf.fr/iperf-doc.php) on cron running every 15 minues for 10 seconds and then dumping the download speed out to a file. that let me graph speed against time, which showed the evening slowdown as a regular occurance.