Possible Over fuelling or something related?

So, I've been trying to diagnose this issue since I've owned the car, I've fixed so so many other things on this car but I can't figure this one out. It's a 2005 2.0 Petrol before I forget to tell you.

Right, so the issue; First of the two main symptoms - Poor fuel economy, I can't say for sure but I think I should be getting more than 25 ish mpg, driving overly conservatively. Second, If I turn the car off and then try to start it back up very soon after it sorta doesn't. It's very hard to explain in text form, it cranks fine, but it sounds like it so nearly fires up then spins a bit quicker for a second then normal speed, then quicker etc, If I hold the key in start for long enough it will start. This issue is a little intermittent, sometimes I can pop out the car to the shop, 5 mins or so and it will happen. Sometimes it can be off for 2 seconds and it won't. It's always possible to recreate the issue but there's no pattern to the procedure required.

I've read on an article or two that flooding can cause this issue (excess fuel in the cylinder - essentially flooding the cylinder), which of course seems to correlate with the poor fuel economy. I've had a couple times where cycling the ignition without starting will cause "Engine Systems Fault" to appear on the screen, what this might have to do with anything I don't know, maybe fuel pump priming on ignition? Not sure. As for the codes, I can only get the codes through the dashboard, as when the issue appears I can't get the OBD to play ball, they are as follows: D900, E196, E510.

Hopefully one of you will be able to shed some light on this, and of course any further information that's needed just ask

Many thanks, Michael.