I don't which power supplies you're talking about, but we sourced some very high quality (and expensive) power supplies. The SunPower bricks were the best - but the SunPower plug tops are basically the same circuit in a different enclosure. The tiny US plug tops also appear to be very good. I don't know about the large US adapters than the eStore came up with, having never seen one.

Since you're in the UK I would assume you're talking about the SunPower units. There is always a higher than average failure rate associated with power supplies due to factors such as mechanically large components (mechanical stress on solder joints etc) and surges and spikes on the mains. Nonetheless you've been unlucky have have three failures - I've never had an adapter fail on me in over three years.

Rob