Setting ble mtu on bx3105

I cannot find a way to change the BLE MTU on your BX3105. I have tried the AT+SRBLE =“name”,mtu command. The device then responds correctly to AT+SRBLE? However data transmission goes into multiple packets with data above 22 bytes long (the default BX3105 bluetooth MTU at connection is 23).
I also get no response when trying to change it remotely from my connected bluetooth device.
I am using code R2.50p (Have tried 2.63 but the “AT+SRBLECFG=” command does not appear to work with this so i cant even set up a session).
Peter Bate