Spaceboy, Angusdog,
Glad to be of help.
The current sensing type has the wire in series with the coil wiring. The Clubman type and the 3" Smiths type have a long white wire with one turn around the plastic former. The 3" Smiths type wire has a red and a black coloured spade terminal. Not sure about the Clubman wiring. Polarity is important here and depends on whether you car is positive or negative earth. If you get this wrong the tacho won't work. The meter is actually an ammeter and if the polarity is wrong it will try and turn in the opposite direction and the needle will just press up against the stop.
I connected mine at the fuse block. For positive earth cars disconnect the wire supplying the coil at the fuse block (White with a Black trace?) and connect the black end of the tacho wire to the fuse block and the red end to the coil supply wire and your in business. For negative earth cars reverse the red/black connections at the fuse block.
On the back of the tacho there should be positive and negative terminals for power supply and a wire to the light (connect to instrument light switch)
On the early 3" Smiths tacho you could set it up for either 6 or 12 Volt +ve or -ve operation.
On the Clubman tacho the printed circuit supplies the 12 Volt supply and I think the instrument light.
I dont have much info re the voltage sensing type but the sensing wire probably goes to the point side of the coil as the voltage will be 0V with the points closed and 12V with the points open. This voltage signal is then converted by the tacho electronics to drive the meter, which in this case may be a voltmeter rather than an ammeter as in the current sensing type. The printed circuit board will still supply the 12V power supply and the instrument light-I think.
Hope this gives some more help.
RonR