I have a set of Goodyear runflats with 18" JCW wheels. My friend at the dealer recommended I rotate them often, bit how often?

 

5K miles unless you notice significant wear patterns earlier than that (and if so, may want to check the alignment.)

 

5k is a good benchmark. I like to do it every 3k, but I'm a bit crazed that way. Just go front to back... don't cross 'em.

If you haven't rotated after 10k, DONT DO IT. MINI worries about snap-oversteer (the rear end spinning around on you) that can be brought on by having worn tires on the rear (when compared to the fronts). It's why they don't recommend rotating the tires.

If you rotate religiously, you are OK to rotate and will get better life from your tires (IMHO). If you are a bit of a rotation slacker, just drive until the fronts need to be replaced and then replace the rears every other set in the front (or until they need to be replaced too).

The standard disclaimer applies: I'm not a professional auto guy, nor do I play one on TV. This is, however, what I believe to be best practice and is how I treat my MINI.