After playing with the I2C bus on the Pi Zero I found that I was unable to change the baudrate with either of the commands

modprobe i2c_bcm2708 baudrate=200000

or

gpio load i2c 2000

Apparently you now need to set the I2C speed in /boot/config.txt

Mine now reads:

dtparam=i2c_arm=on
dtparam=i2c1=on
dtparam=i2c1_baudrate=500000

Of course it does need a reboot to change.

Strangely dtparam=i2c1_baudrate=1000000

won’t work, but that may be due to my receiver and not the PI