'No win, no fee' does not necessarily mean 'No win, no cost to you' nor 'Win, no cost to you'.
If you win, your solicitor's basic fees should be paid by the other side. However, if the other side thinks they are too high, the court can decide that they do not have to pay the full amount. If this happens, you might have to pay the difference out of your compensation.
The success fee is an extra amount you pay your solicitor if you win your case. Success fees in 'no win, no fee' cases are often quite high, and can be as much as double the amount charged in other types of legal case. You may not get all of this back from the other side and, if you don't get it all back, you'll have to pay the rest out of your compensation.
If your case has to go to court, you might need a barrister to represent you, as well as a solicitor. Barristers' fees are usually not included in 'no win, no fee' agreements, so if you do need a barrister, you may have to pay them regardless of whether you win or lose, and this could be very expensive. However, your solicitor may be able to persuade your barrister to charge on a 'no win, no fee' basis.
You may have to pay other expenses for things like experts' fees, accident report fees, official searches, court fees, and travelling expenses. These expenses may not be included in the 'no win, no fee' agreement, and they can be very high.
Before you enter into a 'no win, no fee' agreement, check the terms very carefully for hidden costs.
http://www.adviceguide.org.uk/england/l_claiming_compensation_for_personal_injury.pdf