That's a shame.
My modem was off for a week so that may have been the problem.
Today it was 73/17. Good enough for now until I move in properly.
Was told the full 80/20.
Though I have to say, Plusnet customer service have been great so far. Much better than Virgin's zombies.

if you use the standard ones (assuming you update) you'll soon encounter rubbish rates and wifi signal loss/drops
can't remember why but I'm sure there was a reason
Can't even check speeds or anything yet
.... and my download is the same 5.8meg !!! upload has jumped from 0.8 to 18meg... the BT engineer said he was reading 73meg down.... 
I am thinking the same. Oh well back to plusnet support we go