So, results so far ...
Sync rate from the Technician's portable tester was 39.996 Mbps, who has stolen the 4kbps? The maximum achievable, if not capped, would be 46.667Mbps downstream and upstream 17.667 Mbps.
Testing here has given throughputs of around 36Mbps which is fine and 4Mbps faster than estimated.
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taurus excreta cerebrum vincit