You should be able to get your car serviced anywhere, using MGR parts as Nitrogenics says - in theory.
Past experience leads me to know that if things DO go wrong (and remember we are talking about an MGZR here ) you may have some problems when you go back to your main dealer to sort a warranty problem.
Officially, they are still required to look after it for you of course, but they can make life difficult - no courtesy car available, busy workshops, delays in getting it sorted, etc etc.
They could just allege sloppy workmanship, if even in part contributed to the problem, either way it's likely be much more difficult than it needs to be, just to save a few quid.
Warranty repairs actually pay them very little - they only get trade price back on parts and bare book times on labour, so it's not in their interest to look after a non-customer car, when their mechanics can be working on a "proper" paying job.
They do rob you blind IMO for servicing, but at least you only have to fork out 3 times over 3 years to keep things sweet.