I contacted HP by:
Phone (Message - We're very busy, please call again tomorrow);
WhatsApp (Live agent after 24h wait - "It is a general issue, we are working on it") and;
Twitter (tried to be helpful but no suggested remedies worked).
We tried:
Rebooting holding the power button while connecting power - locked blue screen again;
Updating the firmware from HP website on my laptop - no drivers found;
Tapping the lower left of the printer screen 4 times to access the support menu - ?? blue screen is locked;
Using the mobile HP Support App - no printer found on my network.
Phone the UK Consumer Support line
I managed to get through to the support line where I was given a fault reference number and told to wait until they contacted me.....
Update - I have had an email to tell me to call Support back in 2 days time.