I'm sure PlusNet do have genuine problems with their VMBU reporting but there is also good reason why their reading may be different to many home readings.
I find from a different thread that your BB data when sent over the ATM transport is divided into 53 byte frames which is what the VMBU measures.
If this is correct then there are serious implications:
If you download movies at 1472 bytes per frame then VMBU will round that up to 1484 bytes (53*2 which is less than 1% more.
HOWEVER, if you are a gamer or use VoIP a lot you may suffer more. e.g:
A GSM VoIP packet is 77 bytes (33 byte payload and 44 byte header) VMBU will see that as 2 ATM frams which is 53*2=106 bytes.
This is a whacking 37% more that than you would measure at home, and possibly 37% more than the old VMBU measured, but it is what you use over the ATM network.
G.729 packets would also be measured as 106 bytes even though they are only 64 bytes which is a 65% increase.
Gamers will also suffer from this phenomenon (don't sing the muppet song here) because their packets also contain minimal delta game movement information and will be of similar size to various VoIP packets.
I wonder if there are any games out there who's packets are 54 bytes....