Right, so I suggest following these instructions along with watching the YouTube videos.
First of all you need to remove the key blade and then use this to pop the battery cover off:
Once this is done I used a blade or similar to lever the top chrome piece off, the retaining clip is on the left hand side if looking at the key from the back, but there is no way to access this to ‘unlock’ it so just be a little gentle and it will come free:
The key release slider will come off with this and you’ll be left with the below:
Next is the tricky bit of removing the chrome side pieces, they can be quite tight and although YouTube videos show to lever them from the bottom, I found this was going to mark the plastic, and found the best was was to lodge a piece of paracetamol casing between the chrome piece and retaining clip, then lever with a screwdriver from the top:
Do the same for the other side, you may find the retaining clip is harder to see but it is there.
The next part involves breaking the plastic casing apart as it is glued together from factory, I found the best way was to start at the top end where the chrome top piece will cover:
Gently work your way around until the casing is open.
The next part is somewhat difficult as the circuit board is almost plastic welded to the casing, this needs to be heated and delicately removed and the same done when you’ve swapped the board into the new housing:
Reassembly is the reverse of the above, I glued around the seam of the casing with some loctite gel superglue before reassembling and held it together loosely with the chrome side pieces whilst it dried.
I have the old black key fobs with my W203, its an '06 saffa built one so it must have been the absolute last... one is playing up now despite new batteries so I was wondering if I can use the newer type, does anyone know if its possible?