Are my cable disc brakes knackered?
Usually what causes this (literally no braking, firm lever feel, lever nowhere close to bottoming) is an error in brake setup or adjustment wherein the brake caliper's armature is being pulled to the end of its range of motion before full braking power is reached, or in some cases before any braking power is generated.

This can happen either as a result of incorrect setup or by compensating for pad wear exclusively by way of the cable barrel adjuster without adjusting the stationary pad.

To determine armature bottoming is the problem, see if you can use your fingers to directly push the armature as far as it can go. If possible, try turning the wheel with your hand while doing so to ascertain no braking is happening. Or just undo the brake cable anchor; if the arm was obviously near the end of its range of movement, that's probably been the problem. 

Most mechanical brakes perform their best when adjusted so that the armature starts out near the beginning of its range of motion. The mechanical advantage characteristics of the brake change throughout its travel. So when setting up a brake, always try to get the arm as close to that point as you can while still having all the slack pulled out of the cable. 

Usually adjusting a generic mechanical disc brake with this issue is going to involve undoing the cable anchor, possibly installing new cables if the old one either won't be long enough with the arm starting down in the right spot or has more of a crimp than is safe to have in the system (usually mechanical discs are pretty good about the latter point), loosening the caliper mounting bolts, dialing in the fixed pad adjuster, re-positioning the whole caliper more inbound towards the wheel such that the moving pad starts out closer to the rotor, then re-fastening the cable anchor and doing final adjustments.

For a brake with pad adjusters for each pad, resetting the barrel adjuster and extending the moving pad in may fix the acute problem, although it probably won't be enough to get the arm back to its intended starting position.