OK, having slept on it very little (guess it's much needed practice in sleep-deprivation ) I've gone for the flash. I visited the maternity ward at the local hospital where we had a tour recently, the light in most delivery rooms seemed plenty, however my fiancee has already indicated she's interested in a water birth and it's this room that is particularly dark (no windows). I would hate having a new prime lens and still having to crank up the ISO to 6400 due to lack of natural/any light or missing shots because of low light focus problems (the dreaded 5D2 "issue"). Ideally I should have both, but the budget doesn't stretch far enough this time round so I settled for the portable sunlight option
Let's hope I get time to practice with this new tool, I'll be off reading the flash topics now !!