You could try the Panaracer Mach SK's  ( Slick and Knobbily read 95% area of knobs small spaces ),  fast rolling on tarmac but can handle that kinda thing,  came on my first FS bike strangely.

schwalbe marathon of desired width, weight and price.

awesome tyres. 

I've off roaded on my super motos a lot. Wet grassy descents are possible, as long as you don't need to change direction on the way down

Towpath?? What, do you think I ride a hybrid?!

What is a 23c slick?

Klunk wrote (see)

Mostly road use, but can handle gravel tracks and lanes and steepish wet grassy descents.

Use your old tyres once they're too worn to ride proper off road.

Stick 55psi in and away you go.

23c is narrow light-weight road race tyre. It runs at high pressure and is not regarded as suitable for any off road work.

Mtb road tyres = Continental Contact Sport. It's narrow (for and mtb) slick and runs at 80psi. You can get 'em from wiggle with innertubes included. I use them on my mtb for London-Brighton and Wessex 100  road events.