Thanks again for your reply HarvesteR 
I think your explanation has helped all this information to sink in finally.
I know it probably sounds really daft but the purpose of an accelerometer says it all in the name, right? =P God knows why that one caused me so much confusion. I'm still not 100% as to why you'd need to measure acceleration around anything other than the Y axis to determine if the device is 'falling' as such but I guess that's something I'll have to look into further.
As for the gyroscopes, I can understand why I'll need to measure rotation around the X and Z axis as this will help me to determine if the device is level or not. As for rotations around the Y axis, I think that's where the magnetometer comes into play?
This IMU I found boasts 9DOG http://www.coolcomponents.co.uk/catalog/product_info.php?products_id=462 which i'm thinking may be the best bet for my project seeing it's an all in one solution.
I like the possibility to measure translations from a set reference point, that could be rather handy for 2d/3d wireframing the world and measuring how far the tri-copter has traveled and in which direction.
As I mentioned before, I'm going to make a conscious effort to iron out what I want my tri-copter to be capable of before I start splashing out on 9DOG sensors (78 GBP is a bit steep for a hobbyist =P) but at least you've helped me clear up which tools I'll need for the job. Well, the need for an accelerometer still escapes me but at least I understand it's purpose.
Thank you for your in depth responses, as much as this information is available on the internet you do make it much easier to understand