My principle question is,can I safely use the pattern coat system over the top of this to get the desired finish?

You will need to find a way to protect the ABS from the styrene. Epoxy coating resin is generally the easiest way of doing this. You can also get specialised coatings made for this purpose, like Duratec Styroshield. 

This does tie in with what I have read regarding polyester resins and ABS on model boat forums.I am now wondering if I can get away with making the Epoxy the final finish to the pattern to save on yet more sanding sessions?

There's not a lot of epoxies that polish up too well after sanding, but the XCR resin seems to buff back to a gloss quite nicely from what I have seen. It will be tougher to sand than the pattern coat so it will take a bit of effort, but it looks like it has nice self-leveling properties so you may find it doesn't actually need a whole lot of sanding anyway. 

I would agree that you should well be able to skip a step by not using the pattern coat primer and instead flatting a good finish onto the XCR Epoxy Coating Resin. You'll find that because epoxy is so much harder than polyester that it will take longer to flat and polish compared with the Pattern Coat Primer however, unlike many other epoxies, the XCR resin will polish up to a high gloss, given time. The extra-time polishing the XCR will probably be compensated for by the time and money saved by not needing to use two resin systems to coat it, as you would if you wanted to use the Pattern Coat Primer.

I have already been looking at the XCR resin,I also have a wooden tug hull that is going to need a couple of coats of resin and fine cloth before paint so this product would be useful for that.

That's the route I will go down