I got mine from the flying school, but the phone number which comes with the instruction shhet is 01234 741647. [Moderator: please remove if this offends the rules.]
As for how it works, well, it's a transparent disc marked with concentric circles for the wind and with the 360 degrees of the compass round the edge.
You just mark the wind on it before you fly, using an indellible marker.
When you divert, you mark your line on the chart in the usual way, place your wind dot on the Knightson over the start of the line, aligning the Knightson to true north. You then simply read the heading from the edge of the Knightson by looking at where you line meets the edge, add magnetic variation and there you are.
As for timing, the distance from the point of departure to the edge of the Knightston is 6 minutes. Simply realign the Knightson from the six minute mark for longer diversions, all in multiples of 6 mins.
No mental arithmetic (apart from mag variation and multiplying by 6 to get the time to the alternate).
The only downside is that you must have a different Knightson for different airspeeds. If you fly lots of different types, you might accumulate a collection.