Thanks for your help.
I’ve put the MEL9 inthe first position on my board after my BOSS TU-2 Tuner. It works well in that slot. I have a Pedal Power voltage supply that happily supplies the 9v power for all my pedals and successfully powers up the MEL 9. However, If I leave the pedal board plugged in with the MEL9 attached & plugged in, although I turn the effect button on the MEL9 off, the MEL9 pedal won’t work when I return & turn the amp back on.

Having said that, the MEL9 will successfully power back up again if I disconnect it from the pedal board and then re-connect it!!
Is there a thermal cut-out on the MEL9, please? Is this a problem that I need to return the pedal for, please?