OK. More updates. I fitted the converter chip today. Only requires basic soldering skills. I suppose you could cuse crimp connectors but solder is better :P
Steps were as follows.

Remove strut brace
Remove air filter
remove battery
remove y-pipe [and hose to bov]
remove battery tray and washer bottle
locate speed sensor and unplug [as per picture below]

I cut the plastic shield back to expose the 3 wires. Black, Black with white stripe and yellow. Chip has 5 wires soming out. Cut of the green one as it's not used on our car. The black wire connects to the black wire, the red wire connects to the black with white stripe, the yellow wire must be cut through. The sender end connects to the yellow on the chip and the reciever end connect to the grey on the chip. Then seal everything up with shit loads of tape. I also used heatshrink on the wires I cut and soldered then sealed the whole thing in electrical tape.

Put everything back together. Drive to bottle shop and compare speeds with GPS. Buy beer. Drive home and sit in pool with beer celebrating your success

My speedo has always been out by about 10% so it was still out by approx the same amount once converted. At 100 on the speedo I was doing 150km/h instead of 160. I will reset the needle when I fit the new face so it's correct.

Speed sensor is here. B-34. The haness that goes to it splits up to the ISC as well.


Wiring diagram for the chip.



So now all that remains is for me to print out the new speedo and lay it over the old one and make sure the scale is correct.