Had the visit on Saturday, and the engineer inspected and reterminated a bunch of the connectors, measured the slope of the signal (attenuation at higher frequencies) and replaced the 3db attenuator with an equalizer to reduce the slope a bit, but wasn't really of the opinion that any of this would fix the issue. It's always difficult with an intermittent issue as you can't tell if it's fixed immediately.
Things looked relatively good over the weekend but I suspect that's just lower overall traffic volumes at the weekend. Today we had a full outage while a Virgin Media van was parked further up the road so there is clearly more work ongoing. I'll leave it another few days to see if that maintenance may have changed anything.