There are rules and there’s common sense. Both don’t always line up.
In my opinion the rules give you the option to do what j3 (I’m not really a fighter pilot mate!) and Ando suggest – but would you always want to?!
AIP specifically states that the spot heights on the IALs do not necessarily indicate what’s out there. This is where research and local knowledge come in. There are three airfields into which I fly nearly every day. I reckon I know those airfields and their surrounds very well. Therefore I would be comfortable descending to 300FT above obstacles by day if I had to.
If I was operating into an unfamiliar airfield I would do some research as suggested by JT, but do you always have the time and/or the access to the information? Probably not. Therefore unless the conditions were quite clear below the cloud deck thus allowing me to see any potential obstacles in good time, I wouldn’t descend below circling MDA even by day.
In terms of the original scenario my decision would depend on my familiarity with the said airfield and whether it was day or night.
Day, and familiar with the airfield: Provided I am happy that I can meet the visibility and obstacle clearance requirements, I would continue for a circling approach.
Night, and familiar with the airfield: I can’t descend below circling MDA until on final at night, and I’m already below circling MDA. In this case I would fly the missed approach. (The Young prang is what precipitated the change in the rules for circling at night.)
Day or night and unfamiliar with the airfield: Missed approach.
And if you haven’t considered fuel for an alternate given those conditions regardless what the rules say - good luck to you!