Im in the same situation but on the XL package I get full speeds around 1 am -8 am then around 9AM my speed drops to 1 meg which is just ridiculous.
Called VM to complain india told me it must be my computer I laughed and put the phone down, I then rang a couple of times at lunch time until I got a UK person...
He told me that they would send an engineer out but it is likely my UBR is overloaded, engineer came out checked the signal levels swapped the modem and went
...
Of course this didn't change my speed Ive tried multiple computers so I rang them back up and they have given me a discount because the ubr is completely overloaded they said it could take 8 months before its sorted
.