Not 100% sure what you're looking for but on my phone if you go to logs & zero the counters at the start of your contract month you can keep a track (call duration & data counter) in there.
Not sure that's what you want though?
Sorry for the bump, but do you still need an iPhone app for this?
If so, we're working on adding Tesco Mobile to My Phone Usage in the next couple weeks and would be grateful if you volunteered to beta-test it (as we have only one account to test Tesco Mobile on).