I also say C is AR...and here's a quote from my (shiny brand new Gas Safe edition) Unsafe Situations booklet, just to prove it.
"Open-flued and Flueless Appliances requiring a purpose provided combustion air supply where NONE is provided"..... AR
It would, as has been pointed out, be ID if there were signs of spillage (not mentioned in the question), and there is a case for saying that it is also RIDDOR reportable as you could quite easily argue that if the spillage was caused by the lack of any attempt to provide ventilation then that is unsatisfactory workmanship, which is reportable.
Would still like to hear your thoughts on it though BishBosh...