I brought the car to the meet yesterday and we fiddled around briefly, I installed a new cold start relay and now the fuse doesn't blow (heater works too), but it doesn't engage the CSV either. I didn't have a voltmeter, but will check whether it gets any current at all when cranking this weekend. I was shown a Bosch alternative to the original TTS (VDO) that I'll look for (see photo) it should fit with minor mods, so that might be an option :-)
