Hi,
take off the expansion tank cap and look into the tank. You may see a round float unit that slides up and down an internal post which is part of the tank. If you can see this then you have the latest expansion tank, and this float acts as the sensor.
Underneath the tank there should be a sensor unit which pushes into an orrifice on the underside. Wires then attach to this unit.
If you have the tank with no float, then take out the wire cover in the boot and search for the wires/connector unit that should be tucked away somewhere. If you find this, then you need the latest tank and sensor unit.
If not, then you need the whole kit from Brown and Gammons, I would recommend this over the Lolarm.
Regards
David