**UPDATE** By referring to Cardinal Sin's strip down guide
Grand Scenic Electronic Handbrake Stripdown Plus Photos
in Brakes and Hubs I have succeded in releasing and removing the parking brake.
Anyone in the same position (whose Release Cable doesn't work), follow the parts of that procedure that will enable you to unbolt the brake unit (3 bolts and multi-plug).
Once it's loose, it will remain suspended between the 2 brake cables and you should be able to rotate so that you can remove the torx bolts to lid and carefully pry it off (glued on with gasket seal).
With the mech. now revealed you'll notice a difference in the Cardinal Sin's excellent pics because the 2 peice retraction part has failed to seperate.
With reference to pic 23, you need to spot the top of the actuating arm (marked on the diagram as "Boot Cable Release") BUT as viewed in pics 8,9,12 and 13 where just the top if it is seen.
Using SNIPE NOSE PLIERS get a strong grip of it's "head", where the spring is attached, and pull it out so as to actuate the lever manually. It took several goes for me as my grip failed.
If successful you will hear a loud "rap" as the unit seperates. JOB DONE!
What you you do from there is up to you but I wish I'd have just clipped the brake cables at the caliper and saved time; the driver's side was ruined anyway!
If you wish to follow the advice just given the please proceed using all the appropriate safety precautions and if in doubt, leave it to an expert. Good luck!