I only got warning bleeps when inserting the new bulb , once it was positioned correctly there were no issues and hasn't given any errors since.
Seriously considering swapping out the reversing bulbs now - hoping it makes the reversing camera a little more usable at night as I have to be right on top of my fence posts to see them and the sensors bleep even when I'm miles off them...
please excuse crappy pic - macro photography is not my strong point...
Top - My replacement 42mm bulb which arrived today with the OEM festoon below
Bottom - 39mm LED to fit the vanity mirror with the actual OEM festoon bulb taken from my BW15 GTD - est. 35-36mm...
edit - while the 39mm will work fine in the boot, the 42mm is a great fit and doesn't spin round in the holder - a lot more confident I'm not going to get any warnings now
edit 2 - 39mm bulbs have arrived and they fit the vanity lights spot on - only thing is they measure around 36mm so I'm proper confused now...