Well if you want a very quiet PSU I'd recommend some of the XFX PSUs as they have a feature which turns the fan off when it isn't required to run. The fan will only run when the PSU is under load with some of their power supplies, and the fan will switch itself off when the PSU is not under stress and is not going to overheat. However, the XFX PSUs tend to be quite expensive and generally quite high wattage as they are enthusiast-grade products. You're talking over £60 and around 600+ watts, which may be overkill for what you need.
Generally speaking, most branded PSUs are pretty quiet anyway. It's usually the cheap/generic PSUs that are noisy. Usually the noise in your system is actually more your CPU cooler or the fan(s) on your graphics card(s) anyway, so I'd look into upgrading those first before you look at a new PSU.