Depends on how "semi" you need your slicks.
Mainly road with short cuts off road.
If your riding is almost entirely on-road then Schwalbe City Jets are cracking tyres, or Marathon/Marathon+ if you want seriously heavy puncture resistance (and seriously heavy tyres too).
The ' Schwalbe Marathon' Ones look pretty tough and I think reflected in the price I'll make a note of these
If you want to retain a modest amount of off-road grip whilst still making tarmac less painfull then I think you would be hard pressedto beat Schwalbes Land-Cruiser range.
These look pretty good value for money too and on Chain Reaction Cycles.com they have a small discount off them too so I've bookmarked these. They have a smooth raised centre tread thats ideal on tarmac, but retain enough "knoblyness" for decent grip on anything except in proper muddy conditions.
Am I right in believing that most of the drag comes from the rear wheel? could I get away with just changing this one for a semi slick and leave a fast knobbly on the front!?