When you enjoy a meal, do you like to leave a tip? Most of us assume that, if we do, our cash will find its way into our waiter's pocket. But tipping isn't always as straightforward as it seems.

You might think it's inevitable that your tip goes directly to the person who's looked after you. But in fact, that is far from the norm - and most places divide up tips between all their staff.

Surprisingly, even if you're unhappy with the service, you may be leaving a tip without even knowing it. That's because many places add what they call a "discretionary service charge" on the bill. This means they assume you want to leave a tip of about 12%, so add it on - before even handing you the bill.

Some experts in the restaurant industry experts, like Simon Wright, want to see a far clearer system in place to reward good service. He thinks any charges made should be obvious to the customer.

"Normally a discretionary charge should be visible on the menu," he said, "But it might be on the bottom, and you might not see it. Look for it, and remember the word is discretionary. It's entirely up to you whether you pay that or not."

"I think the important thing is that the customer feels they've done the right thing and given the right reward," he added.

Remember, even if a service charge is added onto the bill automatically, if you're not happy with the service, you have every right to ask for the "discretionary" charges to be removed.

Some restaurants go even further and add a compulsory charge. So, whether you're happy or not, a percentage of the price is added to the bill automatically. But if the charge is compulsory then they should have it written on the menu in the same way that the food prices are shown. And if you don't like the idea of being forced into paying a tip, you might be better off finding a table elsewhere.

These days it's more and more common to tip using your plastic, but this might not be the best option. Because the law states that any tip left on your card is actually the property of the company - not your waiter. That means that when credit card tips are passed on to the staff, the owners can keep up to thirty percent of that money for themselves.

Simon explains why he never tips on his card: "I prefer to tip by cash, because I then like to check, that the staff themselves get it. If I give five pounds for a meal for two that cost £50, I expect the staff to share that amongst themselves. I don't expect the company to have any of that."

As well as keeping some of the tips left on plastic, two companies who own some of the biggest restaurant chains have even been accused of using some of those tips to top up staff pay to the minimum wage.

Simon explained why he's is outraged by this practice: "I think frankly that's been a disgraceful practice, where the customer does not intend the tips to be subsidising low wages in the restaurant industry. They expect their staff to be paid fairly, and for the tip to be something on top," he said.

The law which covers this is set to change on 1 October 2009. From this time restaurants will no longer be able use tips to form part of staff members' minimum wage. But if you tip on a credit card, it's still perfectly legal for restaurants to keep a percentage of that cash, as an administration charge for processing the money back to the staff.

So next time you leave a tip, remember to ask where it's going and who exactly is getting it. You might be surprised by the answer.