Gav,
Iain's system looks very similar to my own.
(Iain - I would be very interested to know where you sourced the sensor bracket as that is exactly what I need?)
Back to Gav - my trigger wheel came from Trigger-wheels.com. I just asked for a wheel to work with an Edis 4 and it does the job perfectly, cost around ?18.
http://trigger-wheels.com/store/contents/en-uk/d5.htmlFitting was quite simple. I drilled and tapped 4 equally spaced holes on the back of the Burtons pulley and fixed the trigger wheel using countersunk cap-head allen screws with loctite. I also machine the pilot hole out so that the wheel was a snug fit on the shaft of the toothed pulley
I'm no engineer but I would have thought that any imbalance introduced by the trigger wheel would be negligible. My engine had received a full dynamic balance and it spins up like a sewing machine with the trigger wheel fitted.
I hope this helps
Jon