Ariston LL43X dishwasher stalls after cold fill with “blocked filter” error

Hi All
I’ve an old Ariston electronic DW and it is reporting the “blocked filter” error immediately after the initial cold fill.
There is no blockage to the drain as (i) it drains before the cold fill, (ii) once the error appears it drains again, completely and (iii) I’ve taken the base off, removed the drain pump assembly and checked the impeller, etc.
I did notice that there was some rust under the float/flood switch in the base indicating that water had sat there at some time in the past and spotted that there might be a little leak around the area of the main motor. It was perfectly dry at the time of my inspection.
I’ve taken the advice of the local service agents and tried to cleanse the machine/check drain hose but it seems that there is no blockage to speak of. The non-return rubber flap on the end of the drain pump is fine and the whole drain pipe is clear. There is no sign of glass or anything foreign in the drain pump or hose.
So I’m wondering could there be another fault?
A pcb/logic problem?
A faulty pressure switch (if there is one on the drain system??)?
Some other fault, in the main drive motor or pump that should circulate the water inside the machine.

The odd thing is that I’ve moved the machine recently (after it started playing up) and when I transported it the first cycle was OK.
Then it stopped working.
Then I opened it up and checked the pump.
The next cycle was OK.
But the next time I’ve tried to use it the fault reappears.
Any useful ideas would be welcomed.

Cheers, Ian