Thanks Pe. The engine is a new MVVS26.
This email was going to include a photo of the sensor and magnet, but its funny, I was looking at the photo and thinking that the sensor must cover the magnet when, (and I'm sure you must have heard the CLICK in Holland when it occured!!!) it occcured to me that all I needed to do was turn the drive washer around and then the sensor would cover the magnet! D'Oh
Sure enough, the engine fired right up!

Thanks for your help Pe.