I use Goldcar or Centauro depending who has the best deal. Both have been fine in terms of quality of cars, good service, and no messing with refunds etc. Also, you need to watch the prices as they fluctuate over time (even for the same hire period). I have found if you book well in advance you don't always get the best deal as they often drop nearer the hire period (I assume because they maybe are under hire targets). And definitley don't buy their additional insurance !! - I use direct-carexcess.co.uk, which cost me around £40 for an annual policy for 2 drivers.
Dave