Android version of MP Bluetooth link

I am using MP on an AK28 Siyi controller. System is connected to a VD32 receiver and CUAV Pixhack flight controller.
Everything was working fine until I added a Lightware LW20 rangefinder to Pixhack serial_4 and now MP will not connect, it just crashes. It complains the Bluetooth interface has stopped/timed out. The fault is very unusual as I tried connecting a laptop with MP direct to the flight controller using USB and everything is fine.
If I close MP on the AK28 and open QGC it connects and works fine.
If I disable the rangefinder, or select ‘analog’ it works fine (except no range)
Any suggestions are appreciated

I wanted to add more detail to my post above. I rewired the LW20/C to I2C and the problem still persists. I used a PC connected to the Siyi AK28 bluetooth and run MP and it works fine. This means that the fault is in the Android version of the MP, and likely something to do with the Bluetooth setup/connection.