£30 a Minute!
According to the Chris Evans radio programme (so it must be true), it costs £30/minute in fuel to hold whilst awaiting an approach into Heathrow.
Let's see, 4 stacks, 6 levels available equals 24 aircraft holding.
Bingo!! £720 a minute disappearing in circles!
Let's hope the well motivated staff don't have to slow things down at all when that new Tower is eventually opened.