I'm reminded of similar issues I blamed on all sorts of complexities. An auto electrician made a house call and started at the battery and found the problem right there with poor connections. I since bought one of those reamers and a jar of vasoline. Perhaps there's a voltage drop turning it over such that letting go of the starter motor then gives the sparks a chance? That could be the battery, the connection, joint to the leads, earthing. Someone recently - I think a Strada Abarth - was telling me how critical the earthing was to getting his car started.
For others to comment on: Is this a case where electronic ign helps? If it was the Abarth then that's electronic ignition, unless that module is voltage sensitive...
I think this was the tool:
http://www.toolstop.co.uk/sealey-ak418-battery-post-terminal-cleaner-4-way-p53164#altsThese are new to me:
http://www.frost.co.uk/terminal-brush.htmlhttp://www.frost.co.uk/frost-battery-terminal-protector-indicator-aerosol.htmlI wish I could be more help with direct advice as to where to put the meter and what to look for as an acceptable or abnormal drop as the RAC man or auto electrician would from instinct.
David