As the weather is so bad, and it's getting dark so early, I've decided to get a turbo trainer. Which is the best type for a mountain bike with slick tyres? And what features should I be looking for?

Cheers!

turbo trainer are good but for more of an interactive ride try a roller. Fall off it and go through the TV set....
also check this link
http://www.ricciltd.com/trainers.html

tacx basic electromagnetic turbo's are exellent giving speed ,distance ,watts etc...the cyclorobx fluid turbo is also good and comes with a video to train to.

Get a magnetic/fluid one or the neighbours/wife will kill you, the fan ones sound like a jet taking off. I have a CycleOps Fluid one that is fine, I have heard good reports about TacX too.