How to choose a set of tyres I have been riding my hard tail on the road for the last 3 years on some knobbly 26 x 2.1 tyres. After just starting a new job and with the need to commute further I am considering purchasing some slicks, however I do not know how best to decide on which tyre to choose. What width should I be going down to and what kind of tread?

I like the look of these http://www.wiggle.co.uk/continental-sportcontact-city-mtb-tyre-and-tube-set but they also come in two widths, is it the thinner the better?
Usually, for a MTB hard tail, a 26 x 1.25 tire is the narrowest comfortable tire to ride. 

It is narrow enough to provide significant benefit to speed and rolling resistance, but not so narrow as to increase the probability of flats and rim damage. 

The continental tires you linked are a good choice. I would probably do the 26 x 1.3 version of that tire.