For information.
The display shows 155 constantly. If you switch the machine off or plug it out, when you plug it back in it still shows 155, If you push any button the display will show ERR for a few seconds also it will not let me enter diagnostics.
Other similar posts have recommended disconnecting the connector block from the display board to the main board (the multi blue wire one), switching the machine on, then off, connector block back on. Usual safety measures.
I can confirm that in my case this did the trick.