B200d brake assist warning
 

The other day I got a message that the brake assist was inoperative and lots of warning lights lit up. I managed to reset this and drove 100 miles. At the start of the return journey the warning reappeared and I couldn't reset it. Today after a few short trips the message was replaced by brake assist reactivated. Something seems to be intermittent.
My car is a 2018 B200d and has done 20,000 miles only. I believe this model has a device behind the star badge (which can be prised off from the lower edge). Maybe the sensor(s) are obscured? But where are these? Are they behind the front bumper or behind the driver's mirror? Could it be leaves or dirt?
My main problem isn't the warning message or lamps but the fact that as soon as the message appears the steering becomes very heavy. Not too bad driving but parking etc it's very heavy.
I have an iCarsoft Pro but it tells me very little. Any ideas?

Since resetting the errors I noticed the right wheel speed sensor was being recorded as faulty so I'm wondering if the other warnings are false and the real problem is a broken wire to the right wheel RPM sensor? Any ideas??