I hope we can add this into a future vCenter update guys, if only to save some other poor sod from going through this.
Yes I know I should have been aware of the time it would take before kicking off, which is definately my fault and I take full responsibility for it, but at the same token I don't think I would have been the first person to get caught out like this. The server in question had mutiple 2TB VMDK's making up a spanned volume, so it really is a show stopper.
In the end we had roughly 3 hours of down time whilst it finished off. It's just the fact that this task can't be cancelled once submitted that I think warrants a warning of some description.
Anyway life goes on.