There are many reasons for the pressure on a boiler to drop.
In this particular case it was down to an expansion vessel that had lost its air bubble.
Every time the heating was used the pressure dropped. This is the classic symptom of a expansion vessel in need of a service.
The solution is pretty straightforward. Open a drain tap and re inflate the expansion vessel. The vessel was full of water which drains out as I pumped it up.
Once the pressure is set in the vessel (normally 1bar). Close the drain tap and top up the system pressure.
The filling loop was neatly tucked away under the boiler. This filling loop has blue taps so you don’t mistake them for anything else.
When the pressure is back at the right level the e119 fault is cleared and the boiler works again.