Phoned sennheiser spare and they dont do one , a curly one is £20.
I bought the above
1M Metre 2.5mm to 3.5mm Jack Audio AUX MP3 Cable Lead
£1.75 on amazon
Remove the concentric rings on the body of the 2.5mm plug , I used a woodworking rasp cos it was handy but a stanley knife or sandpaper would do .
Fits very snugly in the socket have been using for an hour , works a treat