OK, worked example using real numbers (if not real events; I've simplified what my actual bets were) from my first set of bets....
Starting account balance
Coral £5
Betfair £100
(Total £105)
Qualifying bet Teams A v B.
Part 1: Coral
Back A to win/odds 1.5/£5 stake.
Part 2: Betfair
Lay A to win/odds 1.55/£5 stake specified, £2.75 liability (plus future 5% commission on stake if this bet wins).
Account balance after qualifying bet
Coral £0
Betfair £97.25 (100 – 2.75)
Qualifying bet result Team A wins.
Part 1 result: Coral
£5 stake at odds of 1.5 returns £7.50.
Part 2 result: Betfair
£2.75 liability lost. No commission to be paid as no net winnings.
Account balance after qualifying bet result
Coral £7.50
Betfair £97.25
(Total £104.75, -£0.25 from start)
Free bet Teams C v D.
Part 3: Coral
Back C to win/odds 4/£20 free stake.
Part 4: Betfair
Lay C to win/odds 4.1/£14.81 stake specified, £45.91 liability (plus future 5% commission on stake if this bet wins).
Account balance after free bet
Coral £7.50
Betfair £51.34 (97.25 – 45.91)
Free bet result Team C wins.
Part 3 result: Coral
£20 stake at odds of 4 returns £60 (decimal 4 is the equivalent of fractional 3/1; in a normal bet, you’d get three times your stake plus your stake £80, but Coral aren’t returning the stake for the free bet).
Part 4 result: Betfair
£45.91 liability lost. No commission to be paid as no net winnings.
Account balance after free bet result
Coral £67.50
Betfair £51.34
(Total £118.84, +£13.84 from start)