Thank you so much for your advice - really appreciated.
Just did a bit of reading and I hadn't realised that the colour should be different in modem mode. Mine is in modem mode and when I put it into Router Mode the light goes white. So I think that means everything is actually ok with the modem and the intermittent outages are unrelated.
Sorry - I am embarrassed I didn't check more thoroughly!
Thank you again for your advice, you've saved me hours on the phone with support!