I had posted previously that I had another failed MOSFET and surmised it was due to excessive start up current of the Currentlimiting halogen globes.

I had assumed that the FET had failed due as it was passing current despite the drive signal from the Arduino been low.

I have subsequently discovered that some other factor is in play and that the FET has not failed but that something else is holding the gate high. It appeasr that a sudden increase of Drain current causes the FET to "lock" on untill power is removed from the circuit and it resets and begins to work again. - Further investigation required.