When you originally signed up, were you placed on a 2Mbs connection or did you only have 1Mbs/512Kbs? WHat are you paying each month? if £39.99 then you are on the up to 8Mb product.
Regardless of the product name, yes, you will have a speed upgrade up to 8Mbs depending on your line condition. When this will be depends on your product name and when PN can place your upgrade order. The upgrade is free.
For info on this, just read all the threads mentioning MaxDSL or LLU.