Hi there - i know your problem - i too have a black mini - this problem happens to all car - it's just more noticable on black!
have a read through some of the threads on www.meguiars.co.uk
- in their forum for tips - they have a product called clear coat body scrub whic is good - also keep a bottle of their quick detailer in the car to remove the offending poo asap when away from home.
good luck in the quest and check out www.newminiscotland.co.uk
for some local banter too!