Could be any of the above.
I'm with PlusNet now and I was using a Speedtouch USB modem which failed. So I went to my backup and elderly Sagem one and my speed doubled. I kid you not I was flabbergasted, on the Speedtouch I was getting about 1.6 - 1.9 megs and on the Sagem I got about 3 - 3.5 megs.
I've now got a BT router and my speeds are between 3.5 megs and 4.5 megs.
My connection hasn't altered in the slightest with settings etc, all I've done is tried different hardware.
Needless to say I'm delighted.