Solid purple screen on the display with no text and solid purple and purple border lights

Hi there, I am having the same issue with my M1 now. Having changed nothing, I now get a solid purple screen on the display with no text and solid purple and purple border lights. Although the internet is still working (blue light flashing) - I guess it is just a display issue now. Things I have tried:

1. removed SIM and put back in - same issue 
2. factory reset - same issue on restart
3. changing SIM in the M1 - new sim works ok (normal display), but when return to my normal sim, same issue
4. tried my normal SIM in my phone to check it works - it works, so no issue with my normal SIM it seems 

Any suggestions to resolve this? Many thanks