I have 2 different experiences with this bl@@dy system, again from a base in Hunts. Firstly the little LED flashing to red as I was parked (on the phone) in a layby on the A43 when an HGV went thundering past. The slipstram cannot have been that bad. did not even disturb the coffee!
Secondly last week the little red light went on the moment I turned the key and stayed on for the entire journey, and I was being careful, following day on return trip all worked fine. Conclusion is it is not the most reliable piece of kit in the world