Can this be done? If you have a really good ship and Navigator on a vessel, can they lead a fleet through the warp? I seem to remember this being the case (and it makes sense when needing to move a fleet), but are there mechanics for it?

There are no official rules for doing this, but I did make a House Rule to cover it.

When a fleet of ships wants to travel together, one ship (with the best Navigator ideally) is designated as the Lead. The Navigator of this ship makes all the rolls as usual. The rest of the ships then make only the Navigation: Warp rolls to determine travel time, with a +/-10 modifier per degree of success/failure. The fleet then travels at the speed of the slowest ship, and cannot move faster than the speed the Lead ship travels at.

If any of the following ships fail by three degrees or more, then that ship does not count for determining travel times, and becomes separated from the fleet at some point along the journey.

When arriving at the destination, the lead ship emerges first, along with all ships that got the same (or more) degree of success. Other ships roll 1D5 for each degree less than the lead, and multiply this by twelve to get the delay in emerging from the Warp. So two degrees less=2D5, roll of 4 means a two day delay.

Note that successes of 3+ degrees are counted as 3 degrees for this purpose, just as when determining travel times.

Example:

The PCs have a Light Cruiser, and a fleet consisting of a Frigate, a Raider and a Transport are following them through the Warp. All the NPC Navigators have a Navigation: Warp skill of 40.

The PC Navigator makes the Warp Navigation rolls as usual, and gets two degrees of success at the end. This means travel time will be half of the base, and that the NPC Navigators have a +20 bonus to Navigation, for a total of 60%.

The Frigate rolls 02, giving five degrees of success. This would normally put it in the 3+ range and let it travel in a quarter of the base time, but it can't get a better result that the Lead ship and so only counts as having 2 degrees.

The Raider rolls 93- the Navigator makes a serious error, and becomes separated from the fleet during the journey. They'll just have to catch up later...

The Transport rolls 47, for one degree of success. Unless they want to leave the Transport behind like the Raider, the fleet will have to slow down to 75% of the base travel time.

When the fleet reaches the destination, the PC's ship and the Frigate emerge together. The Transport was one degree less, so rolling 1D5 they get 3x12=36 hours. A day and a half later the Transport emerges in the vicinity of the other two ships.

This seems to fit the fluff I've read on moving fleets through the Warp, with the fleets lurking at the edge of a system for days to wait for stragglers to emerge, and there always seeming to be one or two vessels in a large fleet that get lost.

Nope, from everything I've read each ship does need it's own Navigator. Even if the lead ship has a better Navigator to lead the way, the other ships still require Navigators to at least be able to follow the lead ship.

What about:
Astropathic communications within Warpflight?
Ship to ship combat within Warpflight?
Should ship speed have ANY effect on Warp travel?

According to Sam Stewart (I've asked him this before), Astropathic communication is possible in the Warp.

Given several novels/comics for 40k have shown ship-v-ship combat in the Warp, it is possible, but seems like only close combat is reliable, because, depending where you are in the Warp, if your munitions go outside of the Geller Field it's entirely possible it'd just explode, or disappear, or turn into a whale and a bowl of petunias.

As for your ship speed... probably a little, but not a lot. It's your warp engines that are used whilst in the Warp, rather than your normal engines, so the speed of your normal engines shouldn't make a difference really.

The House Rule I gave did state that all the ships had Navigators of their own. For a fleet where only one ship has a Navigator and the rest follow- well, how would the others follow if they had no Navigator aboard?

The closest to what you mean would be literal piggybacking. That is, a ship with neither Navigator, Warp Drive or Gellar Field is taken through the Warp by physically attaching it to another ship which then makes the journey. I've allowed this, stating that the towing ship must be the same size or bigger than the ship being towed. The Navigator also gets penalties to the Navigation: Warp rolls for the trip, based on relative size- I use -40 if the ships are the same size, -10 for a Cruiser towing a Raider, and in between for less extreme size ratios.

You'd have to be careful, though, as any serious Warp Turbulence (eg Warp Storms rolled as an encounter) might separate the ships. And unless the ship being towed only lacks a Navigator, thus having a Warp Drive and Gellar Field of it's own- well, bad news for anyone aboard.

The Gellar Field creates a bubble of Realspace inside the Warp that surrounds the ship. Two ship should be able to communicate using Vox signals normally as long as they're close enough for their Gellar Fields to overlap. In theory these ships could also send shuttles and other small craft back and forth between each other, but good luck finding a pilot willing to fly such a mission...

MILLANDSON said:

"Oh no. Not again!"

-=Brother Praetus=-

What about ramming and boarding actions?
Surely there should be some way to intercept an enemy vessel before it materializes?

(In Death or Glory, a particular Kunnin' Warboss sacrifices a boat full of Weirdboyz to knock out all the Navigators of an entire flotilla)

Generally, from the looks of the fluff, what generally happens is that, if you attack a ship in the warp, you try to knock out the Geller Field to either cause the destruction of the ship in the Warp, or force it out of the Warp, where you follow it, and then destroy the ship. Most cases seem to follow the latter option.