Update:

I didn't have any DIN tools to take the radio out yesterday so I tried making some out of old credit cards using pictures of the actual tool. That didn't work, so I tried some lengths of steel. That didn't work either, and I'd spent about an hour and half trying various things / making various things with no joy so I packed it in for the night.

Went to a local Mercedes mechanic today and he popped the radio out in 3 seconds with the correct DIN tools. Top tip: Get some DIN tools! :P

Once the radio came out I was able to undo the necessary screws (just inside the radio socket, directly in line with the centre air vents) and pull the bezel off granting me access to the hazard switch. The SLK switch is the exact same dimensions so it fit in without issue and that solved the problem.

I transferred the button fascias over but there is a minor aesthetic problem:

The white seat behind the fascia (the part inside the button that's used to actually hit the relevant switch, which the fascia clips over) is slightly different on both buttons. The Crossfire fascias won't fit on to the SLK seats so you have to take the Crossfire seats with the fascia attached and clip them onto the SLK button.

They snap in just fine but they're slightly thinner so when installed the buttons are slightly angled downwards. This also means that when you want to turn the traction control off / use the central locking button you have to push a little harder than normal but it'll do 'til I buy a new Crossfire switch.

The issue with the old Crossfire switch seems to be a spring issue in that the hazard button wasn't popping out when pressed (to turn it on). I prised the button out and it made a small *pop* noise and now it won't stay down when pressed meaning that the hazards would be stuck on if re-installed.

Not sure if I'm bothered taking it apart to find out exactly what the issue is with that - I think it's likely that a piece of plastic has worn down / snapped off so a new hazard switch would be required in any event.

Thanks to everyone for their input and help in getting this sorted!