Resources I found helpful:
http://www.z4-forum.com/forum/viewtopic ... 28#p334131 - the only diagram of the drivers side speaker setup and parts involved (thanks Wondermike)
http://www.z4-forum.com/forum/viewtopic ... 28#p334139 - Next post down with some limited instructions on how to remove the trim (thanks PawnSacrifice)
http://www.z4-forum.com/forum/viewtopic ... 31&t=25306 - Some details on speaker selection and depth.
It was actually a very simple job to do (easier than adding in the door speakers) only a few screws to remove and most of the other trim just pulled off.
The process was:
1 - Pull off the entrance strip, pulling from the front edge seamed easier but it required a good pull to get it off.
2 - You will now see 1 screw holding on the side panel just under the front edge of the entrance strip, remove this.
3 - Remove the tray above the footwell (on the passenger side this is just 2 philips screws, one at the far most edge, on the drivers side there are 3, 1 on the left hand side and 2 on the right).
4 - Remove the side panel (on the passenger side it should just pull off after removing the screw in point 2, on the drivers side there are 2 more screws, 1 for the bonnet release leaver and another hidden under the leaver, also don't forget to unplug the boot release button cable).
As for the speaker replacement them selves it's quite straight forward, I used the following: JBL GT6-6 - 16.5cm - http://caraudiosecurity.com/gt6-6-16-5cm-135w-speakers
On the passenger side the speakers just drop in and there is plenty of room (actually the speaker is a little small and there is room for a 17cm) so I had to use some washers to secure it in place. However on the drivers side there is a funny mounting bracket which will just fit a 16.5cm as a snug fit (again with a little bodging and washer work)... As for depth there is a fair bit of room for these speakers and the ones I fitted had plenty of room, but I would stay mindful of this.
Now for some pictures:
Showing the two screw holes (one behind the bonnet release leaver which has been removed) on the drivers side:
Driver side with all trim removed:
Drives side speaker fitted (with my washer bodge).
As for overall improvement in sound it really is a big difference, sound is so much clearer and defined. I initially thought I had less bass after the swap but actually it's just much clearer now than just a fuzzy thud thud in the background; it also goes much louder without and form of distortion.
Hope this helps someone.