I’ve now done an extensive internet search and can’t find any references to J type self actuating, which leads me to think the problem must be electrical. Loads of threads about them stuck on or slow to engage, none about them suddenly switching on and staying so.
What’s baffling is that it’s stuck in first and second. Implies that power is getting to it despite two switches being off.
Was working perfectly-fast to engage and disengage - so filter and flow issues would, I hope, seem unlikely.
Will look at the solenoid wires from underneath and see if there is an obvious issue before taking out the tunnel (groan!).
Alan