I bought a warranty for my son's car from Motoreasy. http://www.motoreasy.com/
It cost £39 per month but that included an annual full service and MOT (plus anything it fails on is covered inc free retest), but you have to use their approved centres (Many of them are Nationwide Autocentres - quality varies, but my local one seems okay). It also included breakdown cover and vehicle recovery - the only part I tested when my son rolled his car and put it in a ditch, writing it off! When it was recovered there was a £70 charge, because as the small print says, the car is only recovered for free if it is in a suitable position (prob not a ditch).
With the cost of the service, mot and breakdown recovery being worth maybe £200, the remaining warranty is about £22 per month, not too bad I think.
39 was for a 5yr plus car, less than 5 yrs old is £29 per month.
For older / high mileage cars there is a sliding scale excess (In 10% steps per 10K miles above 60K I think).
I was reasonably happy with what it covered, and they were fairly efficient when I dealt with them, but obviously have no claims experience. Sorting out the service / MOT was easy.
The warranty goes with the car, and is transferable to new owners, but that also means as it was written off I still have to pay the remaining installments (interest free) for the year.
All in all this seemed the best value of all those i looked at (some cost this without service / MOT / breakdown), but of course, claims experience is everything.