Hi Talk Talk,
I used to be on Fibre Large and got around 55Mbps speeds, I then downgraded to Medium for a few months and recently upgraded back to large. I was getting 47Mbps, which I thought after a few days DLM would kick in and the speeds would go back to around 55, however looking today they have dropped to 44Mbps.
The max downstream in the router is showing as 65Mbps, so should be able to get above what I'm getting.
Can you have a look at my speeds, and see if you can give them a nudge back in the right direction.
Cheers
Jon