2. Make sure your anti virus is not impacting things, i.e. turn off temporarily or test from another device
Switched off Avast, and it improved download speed by about 50%. ie. from 45mb to 65mb (it cured the "slow start" syndrome and presumably more). We renewed our Avast "Premium" about the same time the fibre went in, and it must have defaulted to "everything" rather than the configured settings we were already using - and we didn't check

However, still not the 115mb we are paying for, so Zen got OR to try again. Went through the text to say they were coming yesterday / watch video / text them to confirm we had watched it routine again - and at the end of it there was a comment that they would only enter the premises if we had NO service. Not happy at that point as we obviously DID have service, but hey ho....
Nobody came yesterday, no phone calls from engineers, nothing.
Tried a speed test last night.... 65mb grrrr
then again this morning https://www.thinkbroadband.com/speedtest/16102024854... and...
bingo! 111mb!
Not only a higher effective speed but the graph shows (most of) the variation has gone away.
Got exited and ran test again.. 66mb again 88mb grrrrr
09-01-2021 14:52:38 (GMT) 88.5 Mbps 18.6 Mbps Zen Internet
09-01-2021 14:50:39 (GMT) 66.1 Mbps 15.8 Mbps Zen Internet
09-01-2021 14:28:36 (GMT) 106.0 Mbps 20.4 Mbps Zen Internet
09-01-2021 14:14:59 (GMT) 111.8 Mbps 21.6 Mbps Zen Internet
08-01-2021 20:16:56 (GMT) 69.0 Mbps 20.0 Mbps Zen Internet
08-01-2021 18:39:37 (GMT) 69.7 Mbps 19.9 Mbps Zen Internet
08-01-2021 18:38:51 (GMT) 65.6 Mbps 19.6 Mbps Zen Internet
This is on a decent HP laptop with Avast "File shield" and "Behaviour shield" and "Web shield" turned off, over a wired connection (all Cat8) with wifi turned off at the router.
Where do I go next?