Booted a different kernel - photon serial bridge recognised just fine, and particle serial wifi got me connected. For future reference the relevant module is cdc_acm.
Then I had the "bad keys" status - cyan flashing with a magenta/green burst. Fixed that following @mdma's instructions from [here][1]:
Using particle, first generate a new key:
particle keys new
Then put the device in DFU mode and flash this new key
dfu-util -d 2b04:d006 -a 1 -s 34 -D device.der
Rebooted the photon and it dropped through to breathing cyan - tinker works so i guess I'm online 
[1]: Photon setup flashing cyan with a "quick red burst" (now orange burst) [Solved] - #18 by Dave