Virgin broadband , anyone having problems with their speed ? I should be on 20 but only gettin half a meg !!?

I am with virgin media ( formerly ntl ) . I have rang em a few times over the last fortnight to raise this issue and they have done all the tests and even sent an engineer out who tested everything and said it was ok !! T o be honest it was working fine whilst he was here , then ever since it is terrible again . I have only upgraded to the faster 20 meg speed a couple of week ago as we used to be on 2 meg but I can never remember having a problem ... It seems to be since we upped the speed ? I was wonderin if it could be the exchange or the line to the house from the exchange ? or maybe with the new 20 meg speed which has only just become available have they overstretched the system ? Is there anyone else having simalar problems ? Especially in the greater manchester area , or is it just me ?? thanks .