I was in Specsavers recently talking to one of the guys about the move. He said that a significant factor in the choice of Open Source was the release of Windows Vista.

Specsavers had been mulling over a change for quite some time, but were stalling because of the cost of familiarising all their staff with a different operating system. Vista gave them the impetus to change because, had they adopted it, they would have had to re-train their staff anyway. Before Vista the choice was to either pay for re-training, or pay for new MS licences. Now the choice is pay for re-training, and either pay more for new Vista licenses, or get Open Source for free. Easy choice.