Nikai: I'm not really a GA type, but for what it's worth:
"...anyone can tell me a little bit about the function of an 'Oil Filter Caution light' in general aviation aircraft such as a C206...." As a general rule, oil filter caution lights (and fuel filter caution lights) indicate an impending by-pass. This gives you time to investigate and rectify the situation before actual by-pass occurs. Check with a knowedgeable engineer or contact the factory for your particular situation.
"...I've been told that the light is activated due to pressure differential..." Correct, a pressure differential switch, and they do occasionally malfunction.
"....and what I am curious about is how critical this type of warning is..." It's very critical. Land as soon as practical and seek technical advice before further flight.
"....and whether there is a period where it may still be safe to fly(ie. get it checked within 'X' hours, or whether its a matter of don't fly it until its checked)..." No, there's no subsequent period during which it's safe to fly, get it checked.
"....I was advised by the engineer who usually looks after the aircraft that the plane wasn't unsafe (via telephone - engineer hadn't seen it since the last check)..." Yes it bloody well is unsafe! He's asking you to fly a single-engine aircraft with an engine oil system caution light illuminated - go figure.
"....however another engineer (who I know has significant experience with these engines) mentioned that these warning lights don't often illuminate and if it has it could be a sign of impending trouble and that it should be checked before flying...." Stick with this engineer, he knows what he is talking about.
"....I know the other possibility is an electrics fault - and I'm sure this will be checked first...." Yes, always go for the simplest potential cause first,
"....however I am curious how an engineer would recommend a pilot deal with an 'oil filter light' warning...." Not really the correct question, the Flight Manual (or equivalent) is the authority for the pilots response to this situation whereas the engine maintenance manual is the authoriy for the engineers investigation and corrective action.
As the engine was overhauled fairly recently, contact the overhauler immediately and explain in detail what is happening and what has been done. Above all, make sure you don't do anything that may invalidate any remaining warranty. Good luck.