1st Car £1500 cash
2nd Car £2500part ex, £500 cash, £7k bank loan (through parents)
3rd Car £4200 part ex, £2000 cash, £3k dealer finance
I found that dealer finance tends to be cheaper when it's smaller amounts borrowed. On my focus to borrow £3k only incurred £300 interest over 2 years. I did however pay it back early and saved about £100 of that. In hindsight i regret taking the £7k loan to buy my second car (a brand new corsa), i got bored of it after the 3 years i spent repaying the finance and then sold it, but then again i was earning enough to pay it and still not worry about money.
If i can manage all my usual outgoings and still pay towards the finance and have spare dosh i wouldn't hesitate at taking out finance for the next car.