How do I stop the pointer disappearing when I type in Ubuntu Mate 16.04

In MATE, the settings has gone for some reason. But if you're referring to the disabling of cursor while typing, that settings can be disabled from gsettings.

Here is the command in gsettings command for MATE

gsettings set org.mate.peripherals-touchpad disable-while-typing false

As @Carolus guessed, it is in the mouse properties. You can reach it from the Mate start menu by searching for "mouse", or by entering mate-mouse-properties in a terminal.

In the mouse properties window, choose the Touchpad tab and uncheck "disable touchpad while typing".

I would advise looking in System Settings (might be called slightly differently in your distro) section of Mouse and Touchpad or it might be called Input devices. At least in Kubuntu 16.04 and Xubuntu 14.04, that is where I found the checkbox "Disable touchpad while typing". Unchecking that gave the desired results for me with the aforementioned 2 flavors of Ubuntu. I hope Mate is similar.