My father-in-law's 1988 240 has a cable for the speedometer. Its possible this is different on European 240s. His is the type with the very large plastic headlamp lenses. If yours has a cable, you can drag the entire cable out, working partly from below the car, or perhaps just replace the inner cable, (which can be done from right above the steering wheel) and leave the cable-housing where it is. If you need to replace the entire cable, you will likely need to work it down from below, because I think the end that screws onto the tailhousing of the transmission is the biggest part of the cable.
I am curious how you know the cable is bad though. Did the speedo needle stop indicating - or is it just the odometer that has quit changing? That is a different problem, well known and fixable with a little effort.
In order to get the instrument cluster & speedometer out from an American 240, you will need to take some phillips head screws out from around the sides/bottom of the instrument bezel, and remove the two knobs on the far left side of the bezel. One is the headlight switch, the other I think is the dashboard lamp rheostat. Both knobs will pull off - but will take a strong grip. With that bezel free, you can work the instrument cluster out - just enough to get a hand behind there, and undo the cables and connectors. You dont need to undo the steering wheel, really.
While you have the cluster out I would advise replacing all of the bulbs in it that look 'darkened'. They are the ones that would burn out in the next couple of years.
Let us know if its just the odometer that isnt working. Its a very common 240 problem.