Problem solved. I didn't get to look at the valve yesterday, as the temp topped at 43 degrees C (about 110 F ) and there was a howling north wind --not the sort of day for pulling hydraulics apart in the field! Had a look this morning and found the valve seat in the solenoid pilot valve fullof bits of what looked like stainless wire. No idea how they got there, butcleaned them out and it works as good as ever.