> >I'm trying to get an explanation from BT and my ISP has sent a request
> >to them to check it!
There are many ADSL line test sets, a representative example is at
http://www.exfo.com/en/Products/Products.aspx?Id=369
just get them to put one on your line and stop faffing around.
A line is just a pair of copper wires, with resistance,
capacitance and inductance, each of which is measurable,
it passes audio frequencies, how well ir carries them,
(which is it's frequency response) is measurable.
There are CCITT specs, which it can be characterised
against.
Openreach and ISP's just keep passing the buck
and give you the runaround, instead of actually
measuring anything.