Nick
Whether you pay for faulty goods or services in cash or by credit card, your rights are against the seller. However, the Consumer Credit Act 1974 provides added protection where you buy goods or services using a credit card, as long as the price of the item purchased was over £100. So if you buy a no-frills flight costing £49.99 by credit card and the airline collapses, you would not be able to claim a refund from your credit card company. (Incidentally, you don't have to pay the full amount by credit card. Say the flight cost £149.99 and you paid £50 deposit by credit card and the rest by cheque - you would still be covered.)
The example I gave about car hire not being covered was as an incidental expense. So if you book direct with a car hire company using a credit card and the car hire company goes bust, you would be able to claim a refund from your credit card company, as long as it was more than £100.
However, say you booked a flight and car hire independently (ie it was not a fly-drive package). If the airline went bust and you lost your flight, you should be able to get a refund from your credit card company for the flight (if it cost more than £100). But the car hire company is still in business and able to fulfil its contract, even though you may now not be able to get there because your airline has gone bust. In this case, the credit card company is unlikely to refund you for the cost of car hire.