Wow, 40 Mbps, that really sucks. How can they possible justify a 300Mbps connection dropping so much speed as being "acceptable".
That's exactly what I said to them on the phone, Infinity 4 users has too loose up to 86% of their speed (down to 40 Mbps) before its classed as a fault where as Infinity 1 (assuming 52Mbps) only has to loose 61% or 50% (for 40Mbps version of Infinity 1) of their speed (down to 20 Mbps) for it to become a fault.
But saying that I have seen our connection 3 times go down to at around 47 Mbps one example shown here, at the time nobody was using the connection at the time of the test and that low 47Mbps only lasted for 10 mins.
I might try and get moved over to a less congested segment, BT FTTP Faults said there is no issues in doing that, so I might hold off to see what the engineer does and says tomorrow.