A quick update, following somebody mentioning website 'fttc-check.alc.im'.
I put my postcode into that website yesterday, and it tells me that there is a 64% probability that my lines are served by one of 2 local street cabinets, and a 36% probability that they are served by the other one. I wonder therefore if this could account for the difference between the 2 lines attenuation. That is to say that my O2 line comes from the nearest one, and the Plusnet line comes from the other one. One of the cabinets is approximately 100 metres from my building in a straight line, while the other is probably more than 500 metres away and takes a more roundabout route. I actually walked down to town yesterday and was able to see both cabinets on my travels.
I suppose to be absolutely sure, I would need to either request an engineering visit as suggested by ?Black Sheep? , or is there perhaps some way of looking up details of which line is fed from which street cabinet from some BT database ?