Hi Niel,
The reason that Time Dilation does not completely stop tick is that it could lead to a lot of different problems with tick events causing errors. So as a measure to prevent the errors, Time Dilation can not be set to 0. What I recommend is get it as close as you can to 0, then switch to paused and use SetTickableWhenPaused to enable tick; I believe this is what Fabrice is mentioning was in UE3. I hope that this helps to clarify.
Cheers!
Alexander