I DID IT! And it wasn't too bad.
As usual, as soon as I finally got around to starting this on Sunday afternoon, after glorious sunshine all day, it started raining!

You need the doors open, so the garage wasn't an option.
Anyway, I waited and finally got started at about 8, and after plucking up the courage, poking a hole in the gromit under the steering wheel was very easy, but I did have a couple of mates assisting. It made things easier to have one person each side of the firewall, pushing, pulling or watching. The pictures in the step by step guide were a real help. Cheer!
Thing is, my 5 metre cables (autoleads) were only just long enough - glad it wasn't in a bigger car. I fed the power and remote down the driver's side, and the RCA along passenger side. I didn't have to remove any trim other than the padding under the drivers seat and the rear seat.
Hiding the wires down the length or the car was made easier by just pushing them under the plastic trim by the carpet. Pulling the seat out is a doddle too - just pull and it pops out, it goes back just as easy by slapping it where the clips are.
I've got to set the levels etc, but I can control all this from my Pioneer HU.
In the end it was definately not too hard and worth the effort - it does help if you've got a practical mate or two helping you though, I'm sure I could have done it on my own, just not as quick.
Cheers Phil and Chris!
Ta.