Is's nearly a year since this incident. The garage did the right thing and changed the gaiters without charging anything. However, only 3 months later, I noticed grease again on the inside of the wheeels. Took a look and found the gaiters had split again. This time because of perished rubber!
The garage, to their credit, took responsibility again and changed them without charge. Now about 9 months after the second gaiter change, everything remains good.
How's that for a decent bit of professional service from a garage?