mbames wrote: That has got it back on track, will monitor for a bit and see how it fairs. I'd forgotten to check HA, as I thought it was turned off!

That's good, there's a possibility that the behaviour will change in a later firmware release but at the moment, the behaviour is that Hardware Acceleration does stop data flow monitor from reading accurately because of the way that sessions are processed when they are handled as hardware accelerated sessions.

fryr wrote: I still have the problem with Data Flow Monitor not showing valid information and my Hardware Acceleration is set to "Disabled"

If the Data Flow Monitor still won't show accurate results with Hardware Acceleration disabled, I recommend reporting your issue to the support team to troubleshoot it.

fryr wrote: I still have the problem with Data Flow Monitor not showing valid information and my Hardware Acceleration is set to "Disabled"

In my experience Data Flow Monitor does not capture IPv6 traffic flowing across the WAN. You can test this by disabling IPv6 on your PC and then downloading a test file again.

Further information:

If I use firmware 3.8.4.4BT (576D17) then Data Flow Monitor does display correct information. A menu Option called Hardware Acceleration appears but the setting is disabled, and remains disabled despite trying to set it to Auto or Manual mode (might be due to having ADSL as WAN FallBack). Throughput drops to around 150/30mbps which would seem to confirm that hardware acceleration is off and hence why data flow gets to see the traffic?

If I use firmware 3.8.4.4 (548006 / non vector), known as STD firmware, then Data Flow Monitor does not display correct information. The menu option called Hardware Acceleration does not appear. Throughput returns to my ISP limited 250/30mbps. Does this indicate that hardware acceleration is on? The command to check hardware acceleration "ppa -v" is not a valid command on this firmware but it is when I run the firmware above (3.8.4.4BT) and it confirms it is disabled.

As my internet is PPPOE to my ISP's FTTH/FTTP ONT (I am not using VDSL) I do not require any vectoring support that is in the firmware 3.8.4.4BT, or any of the other vectoring firmware revisions. So it appears I have the choice of STD firmware that gives me full throughput but no Data Flow Monitor or some vectoring firmware that reduces the throughput but gives me Data Flow Monitor.

If your router isn't going to be connected to the Openreach FTTC/VDSL2 network, you don't need to run the BT firmware.
You could try one of the Vector modem codes such as Vector3, because they support Hardware Acceleration (the non-vector firmware does not).

Now running 3.8.4.4 VT3 (Vector 3) and this is providing both correct Data Flow Monitor information and also the Hardware Acceleration menu option, although it is still disabled no matter what I have tried to enable it. It is also providing the full 250mbps down throughput and 30mbps up - just note that with Data Flow Monitor enabled this does reduce the download speed considerably (so you have to enable to take a peek at what is using the internet and then leave it disabled)