Thanks for the advice everyone ... I do always check for myself and make a decision based on what I need.
The main reason for raising this was really the fact that I had noticed that the advice generated in this email always used to be very accurate (and did sometimes suggest just topping up my PAYG), but something has obviously changed with the system since the Goody Bags have been altered.
As a programmer myself, it seems to me that the code to create this email ashould be quite simple ... suggesting that either the data it is using for the comparison has not been put in correctly or that there is a bug with the comparison logic.
Whatever is causing it, the problem really needs to be addressed so that GiffGaff can't be accused of misselling!