This is my working - but I may have done it wrong!
Halving B's paid out amount of £863.97 = £431.99 so C owes B this.
Halving C's paid out amount of £657.18 = £328.59 so B owes C this.
Halving B's receipts of £276.06 = £138.03 so B owes C this.
Halving C's receipts of £2049.74 = £1024.87 so C owes B this.
Adding together the two amounts C owes B £431.99 + £1024.87 = £1456.86
Adding together the two amounts B owes C £328.59 + £138.03 = £466.62
C obviously owes B in the end - so subtract the two amounts
£1456.86 minus £466.62 = £990.24
Is this correct? I think Margo got the same as us ....... but it's been a head scratching time! I never thought something so simple could be so mind blowing unless it's me getting old!