With slightly aggressive driving, my Beat is now triggering an error code 4 (Crank Angle Sensor) and going into the sluggish limp home mode.

I believe from one or two forum posts and the wiring diagram that this sensor is part of the distributor. Fortunately, I had kept my old distributor from 7 years ago, which had been replaced probably due to my ECU dying due to leaking capacitors, and thereby probably giving a false error code. I had since replaced the ECU.

So with my suspicion that the old distributor was fine (and also being only about 7 years old) the garage swapped back to the old distributor. Yet disappointingly, the error code 4 remains. I have never had this error code before a few weeks ago, so I don't think the problem is with either distributor.

I had a look at the ECU, and being pretty young, it still looks as good as new.

Can anyone confirm that the crank angle sensor is definitely part of the distributor?

My new suspicion is that the problem is with the wiring, worryingly. Incidentally, This summer my tachometer failed, and unlike several years ago, opening it up and lubricating it did not fix it. I see that the tacho is quite close to the crank angle sensor on the wiring diagram, reinforcing my suspicion that its a wiring problem, maybe one of the earths either side of them on the diagram.

I've also asked the garage to check the sensor resistance is nominal, which they'll do tomorrow.

Can anyone give any more suggestions on things to check to find the problem? Thanks.