Not completely, but the infant fare is 10% of the base fare, plus the fuel surcharge, plus an insurance and security surcharge. So 10% plus about another £230 for a long haul > 9 hours.

Can't see how they get to £508 though. It may be that the only fare buckets left now are higher ones than you're booked into, but it seems a bit rich to book your infant into the higher fare bucket.