I'm not saying you are wrong Darragh, just I would put this further down my list of checks. I have never replaced a cat on an astra and seen it very few times in work (I work for an opel dealer by the way).
First thing I would check with a non start, would be immobiliser. If you have an immobiliser problem, the engine management light will flash on the dash. Easiest thing to check and if this is the case the ECU will not allow the engine to start. When this happens you would also lose fuel pressure as the ECU wont turn on the fuel pump relay. Does the engine run for a few minutes before cutting out, or almost straight away within a few seconds? If its a few minutes you can rule out the immobiliser problem as once the ECU gets an immobiliser signal it will stay running. If its only a few seconds, this is because some fuel will be left in the fuel rail and allow the car to start but cut out because the fuel pump isnt running.
Once you have ruled the immobiliser problem out by checking the engine management light, next thing to check would be trouble codes if you can get access to a code reader, will make things easier but I understand not everyone knows someone with one. If you have a problem with your ECU like I explained in my first post you should have the trouble code P0230 - Fuel Pump Relay Contact Problem. Most people suspect the relay as the fault but its not always the case. If you can get the codes read, post them here.
After checking for trouble codes, or if I couldnt check them, I would next check for fuel/spark. Easiest way to check for fuel pressure is not by checking at the pump itself. At the fuel rail there will be a black plastic cover the screws off, under this is a valve which you can attach a fuel pressure gauge to check. As most people wouldnt have one, you can check this by simply pushing in the centre of the valve carefully (have a rag around it to stop fuel spraying everywhere, watch your eyes) and checking if fuel sprays out, if it does, you dont have a fuel problem. If it doesnt next thing to check is for live at the fuel pump. Check with a multimeter/test lamp on the blue/red wire. You will only get a live for a second with the ignition on to prime the pump so you will have to have someone crank the engine while you check this. If you have a live here and still no fuel pressure I would check the pump you fitted. If you are not getting a live or sometimes losing it, see the link I posted.
If you have fuel pressure at the rail, next step is to check for a spark. While the coil packs do give trouble on the 1.4 astras, it usually only gives a misfire and brings on the management light on your engine (Z14XE, it wont on the earlier X14XE), but I have seen a few that caused non start issues. When it does start, you would notice that the engine doesnt run correctly if it was faulty. To check this you need to remove the coil pack and spark plugs and crank the engine with the plugs inserted into the coil pack and the body on the plugs touching the rocker cover for an earth. (Only hold the main part of the coil pack as you dont want to get a shock off it, and also watch your eyes as fuel could come out of the cylinders when you crank it.
After checking these steps (or as much as you can) post back and we can narrow it down for you.
BTW I'm assuming its a 1.4 as this is the most common engine, but let us know as its also available in other sizes which each have their own problems, such as if it was a 1.2 the first thing I would be checking would be the crank sensor as they give touble.