Just below the fuse box is a relay. This is the starter isolator. As well as a switch in the seat base and one on the brake pedal it also has a blocking diode controlling it which I suspect has gone bad.
To test this pull the relay connector block off and short the long arms of the cross (Red and Yellow on mine.)(a paper clip will do) Sitting on the tractor with your foot on the brake it should now work.
Pieter Burke
July 2009