Two weeks and still waiting for Virgin

I wrote recently about my problems with my aging Cable modem and Virgin Media’s email response. It’s now 15 days since the email arrived, so by Virgin’s advice I should be getting the replacement modem some time in the next 13 days. My connection continues to drag it’s heels and fall over if I look at it funny.

Does this work the other way round? I was wondering if I could tell Virgin to allow up to 28 days for my next bill to be paid.