Got the coding done and it works a dream. Since I'm new to coding I first just did the FDL change for the paddles only, but then since it was all going so well, I decided to go the whole hog and change the FA 205 code to 2TB and code the four ECUs. I had a bit more finger trouble with E-SYS doing that, but I eventually sorted it and it's working fine
So - thanks to all on this thread for helping, and a special mention to mokorx for a brilliant DIY doc which is what convinced me in the first place.
PS - I did get lucky with the wire from the SZL to the FEM. So, by buying a spare wheel with paddles, the job was no more than an hour in total.