My 1988 SSE (which I haven't had long) often seems reluctant to start. Sometimes the starter motor turns the engine over really slowly but the engine starts on the first attempt, yet at other times the starter motor works quicker but the engine won't fire till after 3 or 4 attempts and you have to work the gas pedal to get it to fire. When the latter occurs, it will sometimes start on less than six cylinders, with all six working after a couple of seconds. In all cases there is a smell of unburnt gasoline immediately after starting which goes after moving off.

What is likely to be the cause of the problem?

I don't think it is the starter motor as sometimes the starter does work quicker. When the starter failed on my Saturn it got slower and slower each time I started the engine untill it eventually failed completely. This clearly isn't happening here. The battery and alternator appear to be fine too.

You should never have to push the gas pedal on a fuel injected car to start it unless it is flooded. Pushing down the pedal all the way initiates a "flood mode" that cuts fuel.

It sounds like it may be flooded, but you need to post more info. Does it happen more when it sits for a while? Will it start without pushing the pedal.

I'm thinking a nice ignition system tune up sounds like it is in order. Does the starting get any harder with rain or moisture? At any rate, plugs, plug wires, and a delco swap could be considered.

The plugs are easy to do, just go with the normal AC Delco plug @ 0.060". Wires are easy too. The delco swap might be hard since you are in Europe, but I'm sure someone on here can hook you up. It just involves replacing the ICM/Coil packs with a much better, hotter firing version. Buying the wires depends on if you do the swap or not. Get 92-99 wires with the swap, 88-91 w/o the swap.

Thanks for the help so far!

It doesn't seem to make any difference how long the car has been standing for. Like I said, it will sometimes start without touching the gas pedal (albiet with the starter working slowly), but sometimes the starter will work quicker yet the engine will only fire when the gas pedal is pushed at the same time. :?

I'm not sure about the effect of damp as it has been wet weather around here ever since I bought it on wednesday!

Yes in addition to the above mentioned, you might want to check the TPS voltage and check for smooth resistance throughout its operating range. The Idle voltage could be off and the throttle body and IAC valve could need a good cleaning.
Hotter plugs will also help it fire right up.
So many other things could cause this...water in fuel, bad regulator, dirty pin connector at ICM etc.
Its best to start by the process of elimination starting with the standard tune-up.
Its also possible there is an ignition timing issue going on making it spin faster sometimes. A faulty TPS could also cause this.

Another way to tell if there is a problem with your IAC valve is if you have a lopey idle. When my IAC was super gunky on my '89 it idled so bad sometimes it would almost stall. I took out the valve, sprayed it with TB cleaner till it dripped clear. Put it back in and problem solved.

I did a diagnostic check today and got codes relating to the oxygen sensor, TPS, and Park/Neutral switch. I take it the former two are most likely to be giving me trouble starting....

Hehe, yeah I noticed the gas mileage!

Forgot to mention I was also getting codes for the "Quad Driver"... not heard this term before... What exactly is a quad driver? It doesn't seem to be explained in the Haynes manual.....