Well that wasn't too painful. Though the bike was out of action for a week or so, & the kids & wife left the kitchen during both painful sessions (I am not a DIY person).
I got the new shifter (from starbike). The instructions were in German, though the English version is available as a PDF on their order page. You do need to remove the old gear cables, & while a mechanically minded friend did have ideas on how to re-use those, mortals will be advised to buy 2 new cables along with the shifter (doing this cost me a week). Some other brand may work, various LBS insisted that the Rohloff brand cable was the only choice. They admitted to little experience, I have less, so I ordered 2 new cables & some new bayonet connectors (mine had been well salted & one of the headless bolts refused to budge).
Doing the job was easier than I realised, the cables did slip into the right slot. I agonised a lot over which was the "1/down" cable & which the "14/up".
Finally, there is a video on YouTube by none-other than Thorncycles.co.uk, aka SJS, which takes you through the process. The matter of fact tone is soothing, the advice on getting the cable length right works. It's great. Those guys should go into the bike business!