You've not said which package you are on with Pipex, but from a brief glance at their rates page
www.homecalltelephone.co.uk/products/everytime_rates.aspx , the rate for "Local & National" is the same as "ISPs on local & national numbers" at each respective part of the day.
Obviously with the anytime package ALL "Local & National" are included
with Hometime it's evenings and weekends.
But on both plans you would need to disconnect and redial after your 89th minute (Iknow it's 90, but does your clock work like theirs?)
What rate are you paying, what plan are you on, what time of day and how long connected.
I'm asking questions rather than supplying info as I don't think the relevent info exists, as the ISP uses the revenue share from 0844 to help pay for their service.