Or at least I think it is.

Whilst changing tyres this morning, I noticed what looks like CV grease build up on the top of the lower ball joint (these are new) - I can't see a split in the gaiter, but then I didn't spin the hub to check it all the way around.

Assuming it is that, it looks like a fairly straightforward job. I've done a few CV gaiters in the past. Once the disc and hub is seperated, does the CV joing split easy to allow change of the gaiter?

Needn't have worried - no splits in the gaiter.