Sales can be cryptic with their calculations, but I think I can see how they've worked it out:
Code:
£11 Talk weekends telephone
£17.99 2Mb broadband
£19.50 family pack
£-9.75 half price family pack discount
£17.50 sky sports
£-8.50 ongoing discount
...All of which totals £47.74. However, the half price family pack discount will only be for 6 months, so it will revert to £57.49 afterwards.
Now I could be completely wrong, but I'd hedge my bets I'm pretty much spot on with that - ask the person you spoke to for a break down of the costs before you commit to anything.
HTH