After reading some good reviews I've purchased one of these electonic ignition devices. I bought one in England form here Power Spark Ignition
It looks to be the same as this Hot-Spark.

I'm just about to install but have a couple of questions. It says not to connect anything to the -ve side of the coil, but I believe the tachometer is connected here. I've also read that you can connect the tachometer by using a diode.

To those of you who have installed this, how did you handle this? I don't want to fry this and have to buy another!

Just for Info:
I connected the tach back to the negative side of the coil without any side effects.
The tach seems a little more 'sensitive' now, but the car starts first time and idles nicely.
Now I need to take it for a good run as the engine hasn't been used for many years!
Andy