Getting at the bottom hose clip can be difficult but then again everything on a Mini is ;-)
If the clip was fitted in situ - you can get at it with a special long screwdriver - you would probably only ever need it for this one job but it makes it easy...
I bought a cheap one (Black Spur IIRC) and it works a treat. NS wheel off allows access to wiggle the hose off - and catch the fluid in a bowl.
As for the question as to how often. The good book says every 24 months or 24,000 miles whichever comes first.
I guess if you don't know just change it.
Probably best the flush out the system with a hose pipe and have a new hose to hand as the old one will probably split if it hasn't been off for a while. Arrange the clip so the slot is facing up so that the long screwdriver can work from above.
Its a Mini! Get yer knuckles scraped! :-)
Pedro